Comment 3 for bug 521228

I think I've found the problem. First of all, Exaile does read the tags as long as the Musepack files are tagged using APEv2 format. However, most of the Musepack files in my collection are actually tagged using ID3 tag and those files are not recognized.

Here is an example using mutagen (the library Exaile is using to read the tags) with python interactive shell:

from mutagen.apev2 import APEv2
audio = APEv2("/mnt/storage/Music/Pink Floyd/The Division Bell/01 - Cluster One.mpc")

This results in a traceback:
KeyError: "'\\xd0\\x07' is not a valid APEv2 key"

Now 2nd try using ID3 instead:
from mutagen.id3 import ID3
audio = ID3("/mnt/storage/Music/Pink Floyd/The Division Bell/01 - Cluster One.mpc")
audio.pprint()

Results in:
u'TDRC=1994\nTIT2=Cluster One\nTBPM=218\nTRCK=1/11\nTPE2=Pink Floyd\nTPE1=Pink Floyd\nTALB=Division Bell\nTCON=Rock'

Now I cleared the tags using EasyTag and retagged them again which wrote the tags in APE format. Now Exaile is correctly adding the files to the collection.