[itsameta4]

Japanese de-voices vowels in certain contexts adjacent to voiceless consonants. The weakness you're describing is the lack of voicing (vocal cords vibrating).

[gizmo686]

The きや　きゃ (kiya, kya) distinction is not a phonological alternation. For instance, we have ぎゃく (gyaku), despite the fact that /g/ and /y/ are both voiced.

[itsameta4]

I'm aware of this, however he specifically mentioned a weak sound in "sukiyaki" which is due to de-voicing the /i/.