Background Noises volume turning up uncontrollably

When I want to play background noises on my iPhone while listening to music, instead of the background noises staying at a volume of 25 where I want it to be (ie in the background), randomly, the background noise volume raises to 100 until I go back in and change it back down to 25 - except, the volume slider doesn’t even say it’s at 100; it says it’s at 25, but it’s so loud it might as well be bugging up to 100. It’s irritating because I can’t use the background noises as intended.