Whether you are playing the Motion Synth App's onboard sounds or controlling other iOS sound apps, to hear the detail and quality of these sounds you should use either earbuds, headphones, or an external speaker.
If you use an external speaker (i.e. a bluetooth speaker), you should connect your iPhone to the speaker via an audio cable, NOT via bluetooth.
If you send the sound via bluetooth you will notice a substantial delay between when you play the note and when it is heard, which will make playing music very awkward.
Many bluetooth speakers have 3.5mm stereo audio input ports and a 3.5mm stereo audio cable can be bought cheaply from an appliance store or online. Ensure that you get a cable long enough to allow free movement while playing.