I was thinking, while a number of games with tandy support expect tandy video and memory map, so rounding out tandy sound isn't a priority, on the other hand since most everything on the monster card are the monster(tm) versions, dual sids, dual opl2s, cms(dual saa1099s), could stick to the theme as it were with the monster version of tandy, aka topping it off with the tandy dac. Of the simple dacs to add with any remainder space that'd be the one I'd choose if they all couldn't be included since most everyone will have an unused lpt port for covox and disney, there are already projects done for those: Reversing the Disney Sound Source and the only way to get tandy dac otherwise is through a real tandy or dosbox/other. Just a thought when you get to the tail end of the project. Also, have you seen LABS blasterboard project? BLASTERBOARD : A new SB 2.0-compatible ISA sound card Very low noise and has speaker and cd in.
edit: Last time I tried figuring it out, I can't recall any games that supported tandy dac, covox or disney that didn't support sound blaster on mobygames, so all three wouldn't be a priority to me. Something to flesh out I suppose.