Audacity can; a quick search with Google for "audacity mono" finds:
Convert Stereo to Mono with Audacity
Mixing stereo tracks to mono in your Project (a more complex, but complete, set of directions)
I tested the first directions, and they worked OK in my copy of Audacity 1.2.6. Lots of distortion, but play with the gain level and that might help. Hope this works for you!