For old games, a Sound Blaster (1.0, 1.5, or 2.0) will do you just fine. A stock 5170 isn't fast enough to play Wolfenstein 3-D at a fun framerate, so there's no need to bother with a Sound Blaster Pro or better.
Playing MIDI files is different. The very best way to play MIDI files on any DOS-era computer is to put a Roland SCC-1 card in the machine (first GMIDI soundcard) and they all sound amazing. However, if that is too expensive for you, then I amend my previous recommendation and bump it up to a Sound Blaster Pro 2 or a Sound Blaster 16 (or Pro Audio Spectrum), so that you can have fairly-decent MIDI playback over the stereo OPL3 FM chips on those cards.