There are many little matrix synthesizers for the iPhone and iPad. I have about a dozen on my iPhone, but the only one I really ever use is TonePad Pro. I spent an hour or so recording some bits and pieces a few months ago and used one of them for a soundtrack to a little movie I made testing out my GoPro camera on my bike. It doesn’t quite fit the mood of the film perfectly but at least its an alternative to the thrashing or stadium anthem rock that seems to be the defacto choice. If Kraftwerk made mountain bike soundtracks, maybe it would be like this.