Okay, now it seems to be working
Thanks for the help! The libraries look identical to the ones I downloaded from Aminet, but it's happily skipping through tracks now.
The next issue is less serious, but it sometimes (maybe 50% of the time) doesn't connect on startup. Nothing happens when I run IMP3, I have to issue a couple of Break 4 commands, and it ends with a message saying it's unable to connect to the IMP server. It also reports unfreed signals. Running it again then usually connects instantly.