Sorry for the necro but Ive got a few updates for you guys.
The first and most useful is the Studiomux app. it lets you run midi and audio through your usb cable, which effectively turns any of your music apps into viable external synths. You can also use your phone as an FX unit if you configure it properly.
Next up is the Roli app. Its possibly the best freebie music app available. Im not sure if it runs on older phones, but your best experience will be on any of the 6s and onward phones due to the pressure sensitive screen. Its the best implimentation of the pressure sensitivity ive seen so far in any app. The sound quality is excellent and you can expand the sound bank if you so choose. Again, its a freebie, so grab it if youve got the 6s.
Lastly, maschine 2! Ive yet to get ahold of it but the original maschine app was alot of fun for practicing drum patterns on the go and ive heard theyve really stepped up the actual music making capabilities. As with the original, this app lets you sync with your desktop maschine projects.