The onboard speaker had no DAC so doing that in software isn't feasible. Also, unless the sound card as a MONO OUT header (never seen one) you can't really connect a PC speaker there. Assuming it does, however, and it also has a SPEAKER IN, then the cleanest solution would be connect the PC's speaker output to that input, and then connect the speaker to the mono out. If you can get the first part done (adapt a cable), then you might just as well remove the original speaker entirely and put a PC speaker with a longer cable in its place....
Now, if you don't find a sound card that can do that, you can try a more.... hackish solution.
You'll need one of theese
Just cut the RCA outputs, cut the connector of two cheap PC speaker and join the wires (shielding of the RCA cable = ground, the other one is the signal), tape everything and you'll have both speakers internally. If all you want is mono sound, you can cut only one of the RCA outputs, but the sound won't be downmixed, so make sure all software is configured to use only mono sound. Also, you'll need to find an opening to pass the P2 connector through.