The platform doesn't support this at the moment, but that shouldn't
stop you from trying to do it custom! I would suggest adding a record
button around the main source of audio you're speaking of. Once the
user records their comment successfully, you should save it to
SoundCloud with two machine tags: one referencing the master track
(track_id:1234567) and another referencing the timestamp in ms
(timestamp:1234567)
Then you could ask SoundCloud for tracks with the machine tag of
"track_id" that match your track, which also include a machine tag
timestamp.
Finally, you could build a custom player interface that displays the
"audio comments" as needed. I'd suggest a slight fade out of the
master audio when listening to the spoken comments.