Run the check
Compare known speech and the headphone test at moderate volume.
Check the selected output and any EQ, spatial effect, or mono setting.
Compare wired mode or another device; check the Bluetooth microphone-mode guide if relevant.
Read the result
Only one app sounding dull suggests its processing; all sources suggest the output path.
Limits and next step
A listening test cannot measure headphone frequency response in a browser.
Repeat the check after changing one setting or connection. Keep a record of what changed before deciding whether the device needs service.