I've got issues with Tandy speaker support in two Sierra AGI games, currently under DOSBox v0.74-3, but it's a general issue with older versions too.

1) Mixed-Up Mother Goose (1987, game ver. 1.00D, AGI ver. 2.915): Enabling Tandy speaker (via the -t launch flag for the game executable) does indeed make the sound output work as-expected, but the floating letters on the title screen are massively glitched. This only happens when enabling Tandy speaker support; if I stick with the internal PC speaker this bug does not happen.

2) Donald Duck's Playground (1986, game ver. 1.0Q, AGI ver. 2.001): The launch flag does not seem to work with this game. This is by far the AGI game I know the least about, and the DOS version may just be a shoddy port of the PCjr version or something. I have definitely seen videos of this game with proper Tandy speaker support though, so I figured I'd ask if anyone knows anything about it.