You also don't say:
* what quality of audio (i.e. voice quality, low quality music, HiFi [stereo/5.1/7.1/???] hi-sample rate music, ???)
* what kind of audio file (WAV, MP3, ???)
At a minimum of quality, probably the cheapest way, assuming the software for reading the audio file format is already in place and your microcontroller is fast enough, would be to use PWM into an RC filter to some sort of audio amp/speaker (though, it occurs to me, that you didn't even say what kind of audio output you want (e.g. earphone, small speaker, 100W stereo, full 1000W theater surround-sound?!????)
When you ask a question on a forum, think about the people that will be attempting to answer that question, and assume they can't read your mind
