Hey there, DW00,
Thanks so much for using Apple Support Communities. We see that the audio output on your iPhone seems a little low on your stereo system, but we can definitely take a closer look and try to help get this resolved with you.
The first place to start is going to be Settings > Sounds & Haptics > Headphone Safety on your iPhone. It's possible that the built-in Headphone Safety features on your device are kicking in here, which would control the output volume so as to protect your hearing. Since you're using a speaker system, however, that may not be something you're concerned with. This article has more info on that functionality and how you can adjust those settings: Use headphone audio level features on iPhone - Apple Support
Another great article we want to share is Check your headphone levels on iPhone - Apple Support. That page contains some more really good info on how you can keep an eye on the audio output levels and is a good one to have bookmarked.
We hope that this information is helpful.
Have a great day!