I think you have a double problem.
The SP port is intended for a loudspeaker and can drive enough volume for that. Since sidetone is generated locally, you (can) get quite some audio volume.
You can turn the audio volume down (which I expect you have done), but if something happens that will make the speakers do full drive then the volume can be ear splitting and actually cause ear damage.
I made a small jumper cable with 2x 150E resistors in series with the headphone (one on each channel). Now sidetone volume is OK and my ears are protected.
The "150E in series" is what makes an hearphone port, and earphone port. Just look at any radio schematic.
73, Geert Jan PE1HZG
