tldr; It’s normal.
The sound is on the quiet side of things and yes, it’s pretty normal to not clearly hear the sound, specifically the chiptune music you note unless you hold it to your ear.
When I was designing the Arduboy I spent a couple months in a last ditch effort to increase the volume of the piezo by adding some kind of amplifier circuit but the truth is it’s a mechanical problem. I need to fit an actual speaker or create some kind of engineered cavity within the case.
Better sound is high on the priorty list for the next version. When I made the first Arduboy I was happy with a simple beep but very quickly with the development kids people made the chiptune sound and I tried very hard to get what I was already working with louder but it would have required a total redesign and the Kickstarter was already several months late at that point.
So, you don’t need hearing aids.
There are mods though, you can get external audio if you want to wire some connections inside.