While the iPhone and iPad have decent speakers, they simply aren't going to 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 a bit of extra sound.
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. (volume bar is in 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 louder than others.. so you might want to experiment with a few more, and could also try some of the Guitar sounds in Garage Band which are a bit louder.
4. You could adjust your volume boost settings in the pick settings menu of the jamstik+ app. We wouldn't recommend moving this around much, but you could try moving them up by 5-10 units.
Hope this helps!