Hi bmarshalla,
Thanks for reaching out using Apple Support Communities.
We understand you're experiencing lower volume than expected when using a set of wired headphones. Happy to help with this.
You mention testing the earphones and connecting adapter with other iPhones. Have you tested your iPhone with other headphones?
You also mentioned adjusting any settings you can find. Just wanted to point out one that is sometimes missed related to volume limits.
Limit the headphone volume
Take a look in Settings > Sound and Haptics > Reduce Loud Sounds. If this is set too low it could impact the volume.
Is this happening with all applications, or just certain ones?
Feel free to reply with these details and we will try to assist. Take care for now.