You can adjust volume on both the transmitter Oasis Plus and headphones.
Short press " +" or "-" button once. Press and hold is not correct.
1). Headphone volume button ONLY works when the headphone works under Bluetooth mode. If you use the headphone under wired mode, the volume buttons will NOT work.
If you use HT5009 as a set, or use the headphone with your cellphone / PC as Bluetooth headphones, you can tap the volume button and adjust the volume.
2). Make sure you TAP the "+" "-" but NOT press and hold it. Make sure you tap the "+""-" on two ends NOT the MFB button in the middle.
3). Hold the headphone in hands, buttons horizontal, then quick click the volume buttons 10-15 times and see whether it works.
If still no luck, please contact firstname.lastname@example.org.