You're recording the output of a 5150?? You mean the original IBM PC? Have we forgotten how the PC generated audio, until real audio cards were generated? They just toggled a line connected to the speaker-- thereby pumped square waves at it. Squarewaves are rich in harmonics, and sound very harsh. They were reliant on the fixed response of the speak to smooth it out mechanically. Recording that properly will be pretty hard. You need an RC filter to try and smooth that out, and I'm not sure what that would look like. Probably a capacitor from the hot lead to ground, with resistor in the hot lead before the cap. It's still going to be harsh.