Check the following points.
Headphone specifications
The Aerophone’s PHONES jack outputs in stereo. On headphones that only support mono input, sound is only heard on one side. Connect stereo headphones to this instrument.
Adaptor plug specifications
Check the specifications of your adaptor plug when using it with stereo 1/4” phone type headphones and connecting to the Aerophone. Sound is only heard on one side when using a mono type adaptor plug.
Connect the headphones to a different device (with a stereo mini plug)
If you can hear sound from both sides when connecting the headphones to a different device such as your smartphone, there may be a problem with the Aerophone. On the other hand, if you can only hear sound from one side when connecting to other devices as well, check whether your headphones are malfunctioning.