Those liberated souls (the inhabitants of the Vaikuntha planets) are engaged incessantly in singing songs mentioned in the Sama Veda. All of them are personifications of the five Upanisads
SB Canto 2
The Supreme Personality of Godhead is the controller of immortality and fearlessness, and He is transcendental to death and the fruitive actions of the material world. O Nārada, O brāhmaṇa, it is therefore difficult to measure the glories of the Supreme Person.
The glories of the Lord, in the transcendental seventy-five percent of the Lord's internal potency, are stated in the Padma Purāṇa (Uttara-khaṇḍa). It is said there that those planets in the spiritual sky, which comprises the seventy-five percent expansion of the internal potency of the Lord, are far, far greater than those planets in the total universes composed of the external potency of the Lord. In the Caitanya-caritāmṛta, the total universes in the external potency of the Lord are compared to a bucketful of mustard seeds. One mustard seed is calculated to be a universe itself. In one of the universes, in which we are now living, the number of planets cannot be counted by human energy, and so how can we think of the sum total in all the universes, which are compared to a bucketful of mustard seeds? And the planets in the spiritual sky are at least three times the number of those in the material sky. Such planets, being spiritual, are in fact transcendental to the material modes; therefore they are constituted in the mode of unalloyed goodness only. The conception of spiritual bliss (brahmānanda) is fully present in those planets. Each of them is eternal, indestructible and free from all kinds of inebrieties experienced in the material world. Each of them is self-illuminating and more powerfully dazzling than (if we can imagine) the total sunshine of millions of mundane suns. The inhabitants of those planets are liberated from birth, death, old age and diseases and have full knowledge of everything; they are all godly and free from all sorts of material hankerings. They have nothing to do there except to render transcendental loving service to the Supreme Lord Nārāyaṇa, who is the predominating Deity of such Vaikuṇṭha planets. Those liberated souls are engaged incessantly in singing songs mentioned in the Sāma Veda (vedaiḥ sāṅga-pada-kramopaniṣadair gāyanti yaṁ sāmagāḥ). All of them are personifications of the five Upaniṣads. Tripād-vibhūti, or the seventy-five percent known as the internal potency of the Lord, is to be understood as the kingdom of God far beyond the material sky; and when we speak of pāda-vibhūti, or the twenty-five percent comprising His external energy, we should understand that this refers to the sphere of the material world. It is also said in the Padma Purāṇa that the kingdom of tripād-vibhūti is transcendental, whereas the pāda-vibhūti is mundane; tripād-vibhūti is eternal, whereas the pāda-vibhūti is transient. The Lord and His eternal servitors in the transcendental kingdom all have eternal forms which are auspicious, infallible, spiritual and eternally youthful. In other words, there is no birth, death, old age and disease. That eternal land is full of transcendental enjoyment and full of beauty and bliss. This very fact is also corroborated in this verse of Śrīmad-Bhāgavatam, and the transcendental nature is described as amṛta. As described in the Vedas, utāmṛtatvasyeśānaḥ: the Supreme Lord is the Lord of immortality, or in other words, the Lord is immortal, and because He is the Lord of immortality He can award immortality to His devotees. In the Bhagavad-gītā (8.16) the Lord also assures that whoever may go to His abode of immortality shall never return to this mortal land of threefold miseries. The Lord is not like the mundane lord. The mundane master or lord never enjoys equally with his subordinates, nor is a mundane lord immortal, nor can he award immortality to his subordinate. The Supreme Lord, who is the leader of all living entities, can award all the qualities of His personality unto His devotees, including immortality and spiritual bliss. In the material world there is always anxiety or fearfulness in the hearts of all living entities, but the Lord, being Himself the supreme fearless, also awards the same quality of fearlessness to His pure devotees. Mundane existence is itself a kind of fear because in all mundane bodies the effects of birth, death, old age and disease always keep a living being compact in fear. In the mundane world, there is always the influence of time, which changes things from one stage to another, and the living entity, originally being avikāra, or unchangeable, suffers a great deal on account of changes due to the influence of time. The changing effects of eternal time are conspicuously absent in the immortal kingdom of God, which should therefore be understood to have no influence of time and therefore no fear whatsoever. In the material world, so-called happiness is the result of one's own work. One can become a rich man by dint of one's own hard labor, and there are always fear and doubts as to the duration of such acquired happiness. But in the kingdom of God, no one has to endeavor to attain a standard of happiness. Happiness is the nature of the spirit, as stated in the Vedānta-sūtras: ānandamayo 'bhyāsāt—the spirit is by nature full of happiness. Happiness in spiritual nature always increases in volume with a new phase of appreciation; there is no question of decreasing the bliss. Such unalloyed spiritual bliss is nowhere to be found within the orbit of the material universe, including the Janaloka planets, or, for that matter, the Maharloka or Satyaloka planets, because even Lord Brahmā is subject to the laws of fruitive actions and the law of birth and death. It is therefore stated here: duratyayaḥ, or, in other words, spiritual happiness in the eternal kingdom of God cannot be imagined even by the great brahmacārīs or sannyāsīs who are eligible to be promoted to the planets beyond the region of heaven. Or, the greatness of the Supreme Lord is so great that it cannot be imagined even by the great brahmacārīs or sannyāsīs, but such happiness is factually attained by the unalloyed devotees of the Lord, by His divine grace.