First, please be assured that this issue is unrelated to the Musiah app. Rather, it is an issue / setting on your device.

There are a few things it could be…

Separate speaker / headphone volumes
iPads have separate volume settings for speakers and headphones. So, for example, if you turn the volume all the way down to zero (or mute the sound) while using the speakers, you’ll still be able to hear the sound if you connect headphones. However, if you disconnect the headphones, there will be no sound as you are now back to the muted speaker volume.

Silent mode
If you swipe down from the top right corner (or whatever method is applicable on your iPad to open the control panel), you bring up a menu. In this menu you can silence the iPad. It’s a bell icon, and it will be red if it’s on.

Recently, we had a customer from whom this was the cause of the issue. For some time, he overlooked this suggestion even though it was included in our initial advice. In the end, after a lot of wasted time and effort on his part, he discovered that this bell icon / silent mode was the cause.

In his words:
I remembered that Musiah was built using Unity; I looked that up because I don't recall hearing about it before and learned that it's a cross-platform framework for game development.

Then, I remembered that you had written something about a bell icon, and I went looking for that. Because I like computers to be silent (I can't stand Siri, Alexa, or their brethren) and to never notify me, unless I ask for something, I had set my notifications to off and turned on silent mode, the bell.

I flipped the values of each of those separately. Notifications being on or off did not change Musiah's performance, but clicking that bell, thereby turning off silent mode, allowed Musiah, the game app, to make noise via the built-in speaker!

Then, I retested it, and Musiah the game works as well as GarageBand. [Note: previously only Garageband sound had worked because the iPad treats Garageband as an app but Musiah as a “game”]

Silent ringer
Some iPad models, e.g. iPad Air, have a switch on the side for silent ringer. If this is on, then it can cause a no sound issue. One customer found that the iPad does play music and videos with that switch on so they didn't realize it was on. If you flip it back to ring you should hear sound in Musiah.

Factory reset
Lastly, we recently had a customer with a similar issue where, in the end, what fixed it for him was doing a factory reset on his device.

Unfortunately, we can't really offer any more suggestions than the above as the issue is a) outside the control of the app and b) something we cannot test for without your exact model of iPad which we most likely don't have.

Hopefully one of the above thoughts will lead you to the solution, but if not, perhaps it may be a question for Apple(?)