How to resume normal iPhone volume after notification causes volume to get stuck in lower volume mode?
If my understanding is correct about what you're asking, I don't think this is supposed to happen. When I am playing music (regardless of whether it's through headphones or speaker) and get a text/push/alarm, etc., the music volume will lower temporarily to allow the alert to be heard. As soon as the alert is over, the volume returns to normal. If your phone is doing something differently, you might think about doing a restore of it via iTunes.