Tandy SPU

Often called "Tandy 3-voice", this is not entirely accurate. There was a 4th voice, a noise channel. And Tandy sound integrated the PC Speaker that could be used as a 5th voice (square wave). This arrangement is virtually unique to Tandy 1000 machines and some Tandy specific games used 5 voices. These game will probably be missing the '5th voice' (speaker noise) if played on other hardware and the "Tandy Sound" or "Tandy 3-voice" option is selected.

Voice 1, Square wave, Independent Volume, ABSR can be simulated using the CPU
Voice 2, Square wave, Independent Volume, ABSR can be simulated using the CPU
Voice 3, Square wave, Independent Volume, ABSR can be simulated using the CPU
Voice 4, Noise, Full Volume
Speaker, Square wave, Full Volume only

This is not the same as the IBM PCjr's sound system that could use the PC Speaker OR 3-voice and hardware disallowed using them simultaneously. This xor option was not enforced in the base hardware of other IBMs and compatibles but it was often hardware enforced by add-on sound cards that had a PC Speaker pass-through. A sound card with a PC Speaker pass-through that would play speaker simultaneously with other sound modes was really the only good option since sound from the internal speaker and external add-on speakers simultaneously would be quite dissonant; but not all cards will simultaneously mix with the speaker. So, software also rarely tried to use the speaker with other modes.