This is a very simple LFO generator I made to modulate my Microbrute. Alas, the output voltage of my Samsung Galaxy goes up to about 0.3 V and Microbrute needs 5, so you will need some sort of amplification or bear with the subtility of modulation amplitude (goes up and down to cover about a fifth in pitch, though). Luckily there seems to be no output filters on my Galaxy to mess neither with noise nor the low frequencies.
Lame screenshots, default android GUI design, waveforms: SIN, SQR, TRI, SAW and S&H / noise generation for now... Hopefully someone with more knowledge will make something more useful, maybe with MB-gate-out to Android-mic-in to retrigger LFO, etc.
(works with my 'trusty old' Samsung Galaxy S i9000 - android v 2.3.3 and Yarvik Xenta tablet - android v 4.1.1)