The avast! window is not refreshed for every file, but rather every second or so. Couldn’t it be the case that it scans all of them, and it just looks like it picks only some?
You can go to program settings, turn on the creation of the report file and check the “OK files” to be included (note that the report file may get quite big) - then, you’d see what files are really scanned.

Maybe… but just maybe… could there be a difference that some of the MP3 files have a WAV header in fact? Though I’m not sure if it should make any difference to avast! anyway…