I was flying in an MTO yesterday which has no sidetone when you press the PTT button, so the pilot cannot hear through the headset what he's actually saying to ATC.
And if he turns his helmet volume to max, he gets feedback through the headset.
FlyCom helmet, ATR833 radio, MTO
If you have the same equipment and it works well, could you possibly share your settings/configuration?
I have an ATR833 in my Calidus, with one Pooley headset and one Sennheiser headset. The sidetone volume (description added into ATR833 Manual version 3.01, page 14, section 2.4.6) is controlled by the intercom volume (and if you want to alter one but not the other, you have to change the microphone sensitivity, then readjust the intercom volume). I can't diagnose any further without more information - does the set up work correctly with two headsets (i.e intercom works in both directions and volume is OK)? Does it change if you swap the headsets? If the pilot talks to ATC, does the passenger hear what the pilot says? As for your comment about feedback: interesting but is logically incompatible with there being no sidetone. Is the feedback still there when the helmet is being worn? If so, it is not audio feedback (the audio output from the speakers cannot get to the microphone if somebody's head is the way). If there is feedback (screeching) when the headset is being worn then that is electronic feedback being generated in the radio's amplifier, possibly from an unshielded cable or a broken earth connection. Don't allow the feedback - apart from what you can hear being deafening, unpleasant and possibly damaging to the ears, there is also the consideration that the feedback may (almost certainly will) extend into frequencies beyond the hearing range and could ultimately damage the radio set.
Maximum helmet volume should not be required (!) - I have my headsets at approx half volume and the ATR833 at VOL 10 (from memory). The most critical setting is the squelch (I can't remember the number) - I set it so that when taxiing, the headset is quiet and I have to speak loudly (or go "TA!") to make the intercom switch on. I have it like that because when I am flying, the extra engine and airflow noise almost but not quite triggers the intercom - so any speech between pilot and passenger triggers the intercom quickly (and we don't have to shout).
Also from bitter experience, don't leave an unused headset (microphone) plugged in: on one flight without a passenger, the spare headset was hung on a hook and the lack of a passenger's body to dampen the noise (inside the Calidus cabin) meant that after takeoff, the spare headset's microphone triggered the intercom for the whole flight. After a noisy flight, I found I could not hear the ATC at my destination. I solved that problem by climbing a little before each transmission, then powering down to idle and descending while transmitting/receiving. The engine noise on idle was low enough to kill the intercom and peace was - briefly - restored.
Hope this helps. Willing to help further if required.