In this excerpt from the Manava Shrauta Sutras, a text associated with the Yajur Veda, a certain ritual for worshiping Shiva is described. As part of this ritual, the five-headed form of Shiva is supposed to be worshipped. For those who don't know, the five heads of Shiva are known as Sadyojata, Vamadeva, Aghora, Tatpurusha and Ishana, and according to the Shaiva Agamas they correspond to the five gods Brahma, Vishnu, Rudra, Maheshwara, and Sadashiva.
But the Manava Shrauta Sutras describe the five heads slightly differently:
With the following five verses the invitation of Rudra is performed. With the verse: "Newly born etc." ... [Sadyojata] is the seer, Brahma his deity, the trishtubh his metre, the swan his riding animal, his face westward, the earth his element[.]... With the verse: "Wealth today, O Savitr, etc." ... Vamadeva is his seer, Vishnu his deity, the trishtubh his metre, Garuda his riding animal, his face northward, the water his element[.]... With the verse: "Homage to Rudra..." [Aghora] is his sage, Rudra his deity, the brihati his metre, the bull his riding animal, his face downward, the heat his element[.]... With the verse "We worship the primeval spirit..." [Tatpurusha] is his seer, Surya his deity, the gayatri his metre, the horse his riding animal, his face eastward, the wind his element[.] With the verse "Him who rules" ... [Ishana] is his seer, sound his deity, the brihati his metre, the tortoise his riding animal, his face upward, the ether his element[.]
As expected, Sadyojata, Vamadeva, and Aghora are said to correspond to Brahma, Vishnu, and Rudra. Tatpurusha, on the other hand, is associated here with Surya the sun god, not with Maheshwara. But I'm interested in the fifth figure mentioned. My question is, who is the deity that Ishana is said to correspond to, the one referred to as Sabda or "sound" who rides on a tortoise?
Is the word "Sabda" used in the sense of Sabda Brahman, a concept found in Kashmiri Shaivism according to which vibrations of sound are equated with the supreme Brahman? In any case, as I mentioned above, according to the Shaiva Agamas Ishana corresponds to Sadashiva, although considering that there was a difference in the Tatpurusha case that's not a guarantee.
Are there any scriptures that describe Sadashiva as having a tortoise as his Vahana? If so, what is the reason for that?