I have about 20 files of music in the format .aa3. Which Linux-compatible program reads that format?
Open a terminal and execute this command to install necessary codecs to play
Then you can open and play files with standard player. Such as Rhythmbox or totem.
OP says the method doesn't work.
If this method doesn't work, I guess the files are DRM protected. In simple words, DRM protection is used to prevent piracy of Digital contents (there are other usages also) such as Music, Videos etc. You need a key to decrypt or read the digital content which is only supplied to you by the Vendor.
Check these Links to know more about DRM protection: