No, this sound card is definitely not intended for such a slot that expands the ISA slot to 32 bits.
This is a Compaq-specific sound card meant for a Compaq-specific ISA slot extension. There is no more than the six pins for Compaq-specific audio extensions.
Based on a few pictures of the card, it simply routes some audio signals between card and backplane. ...
Were those slots manufacturer-specific or was there some kind of (unofficial) standard?
(Well, there was EISA, but I guess it's safe to assume that this question is explicitly not about EISA)
What additional signals were available in these slots?
Most likely D16..31 and A24..31. Plus maybe BE0..3 - and that's where the main issue ...