HyperCard 1.2.5 is not fully compatible with System 6.0.7. If you never "play" a sound, it works fine, but because Apple can't guarantee that sounds will never be played, the software is listed as "incompatible".
Claris suggests turning the volume all the way down; in effect, turning the sound off.
Here's another workaround for turning the sound down: add the following two handlers to Home's stack script:
on play
--stops most play commands getting through to HyperCard, it's unlikely
--that a stack would have used "send play ... to HyperCard".
end play
on beep -- flash the menu bar instead of beeping
type space with commandkey
type space with commandkey
end beep