I have discovered that:
- sounds attached to animated object are not considered by subsequent animated
objects (if they are set to start "after previous", these start immediately
after the previous animation is over, even if the sound attached to the latter
is still playing and will go on for a while);
- the automatic slide change, instead, does consider the sound attached to the
last animated object on a slide. In fact the slide will change only when this
sound has finished playing, even if you set the delay time for automatica slide
change to 0.
I think this second behaviour is preferable: most of the times you want the
comment on an object to end before next object arrives: now to do so you have to
check the duration of the audio and manually set the delay for the next
animation accordingly. 
It would be better to have the next animation automatically wait for the
previous attached sound to end before showing, just like the slide change
mechanism. If you want the next animation to start before the sound will last,
you just set a "together with previous" flag and - possibily  - a delay to
postpone that action.