The affix ya comes in the chandas (Vedas) after the word soma with the force of matup,
The force of the affix मयट् is that of ततः आगतः 4.3.74 and 4.3.82, विकारावयव 4.3.134 and 4.3.143 and प्रकृत 5.4.21. The case in construction will vary according to the sense. Thus सोम्यं मधुः पिबन्ति = सोममयः मधुः ॥,