While the iPhone and iPad have decent speakers, they simply aren't going to be the same as headphones or connecting through an audio cable to an external speaker like we recommend. However, there are a few things you can do to boost the sound a bit.
1. Make sure your device volume is turned all the way up (via physical buttons).
2. Make sure the app volume in the Jamstik app is turned all the way up (the volume bar is on the top right above the sounds selector).
Sometimes it's easy to miss as one or the other might not be turned up all the way.
3. Some sounds are naturally louder than others. Experiment with a few more and try some of the guitar sounds in GarageBand as well - most of these are even louder yet.
4. Adjust your volume boost settings in the pick settings menu of the Jamstik app. We wouldn't recommend moving this around much, but try moving them up by 5 to 10 units.
We hope this helps!