My thoughts exactly! I know that I am not using my n810 to its fullest, but with some apps like these.... Hmm, I know I'd get to be everyones friend around here with thier boombox cars!! LOL
Not to mention also, Sure would compliment my huge rolling media center!!!
I would also use apps like these. I actually have Tuner installed, even though I don't have any instruments to tune, just because I like to play with the audio frequency readout - some real audio toys would be great!
I've seen example source for a DTMF tone generator for the DSP, might also be worth doing your spectrum analysis on there too with a short time Fourier transform or the like.
I'm happy to help people to get started with the DSP if you decide that's the way to go.