Our apologies for any confusion MidwestMacMan,
Was any music that’s not downloaded on the device being played at that time? If so, that can contribute to the data usage. Downloading the songs to the device while connected to Wi-Fi can prevent data usage at the time of download, and prevent future dat usage when playing music.
With the data usage itself were you able to identify Music as the app using the data in Settings > Cellular (Mobile Data), or noticing overall usage on the billing statement? Since noticing this usage have you verified iOS is updated? If not, updating is a great step for reaching a resolution. You can find the steps for updating in the following article.
Update your iPhone, iPad, or iPod touch
If the iPhone was connected to a Wi-Fi network there’s also the possibility that Wi-Fi Assist was enabled. When this setting is enabled the iPhone will automatically connect and use cellular data over Wi-Fi when there’s weak Wi-Fi signal. This can be turned off in Settings > Cellular (Mobile Data), then scroll down and tap Wi-Fi Assist.
More information about cellular data usage and Wi-Fi Assist can be found int he following articles.
Use cellular data on your iPhone or iPad
About Wi-Fi Assist
If you continue to have issues after updating and turning off Wi-Fi Assist. The next step would be to contact Apple using the Get Support link previously provided.
Have a great weekend!