Biggles:
If you've swapped ou the radio, headsets and antenna cable there is not much else.
This could be a simple case of too much mic gain. If there is a setting in the radio for mic gain, try adjusting that to see if it improves things. If you are using non-ANR headsets, you will get noisier transmissions than using ANR headsets. However, ANR headsets sometimes have powered mics which might overwhelm the input to the radio.
Also, you didn't mention if your aircraft has a separate intercom system. If it does, try adjusting the mic gain and output levels to the radio on that.
You listed several aircraft on your profile, but I'm guessing the Jab is the only one with a microair radio. Was the radio always this bad or has it just started? (Or have you just found out that you probably always were transmitting garbled?)
Another possibility is a poor ground connection to the radio. This will be a bit harder to check as you will have to dig into the wiring and visually inspect the wiring to the radio. I've forgotten how the Microair radios are connected, but I think it is with a computer style DB25 connector. That being the case, there will probably be 2 or more pins used for ground connections into the plug. They should ALL have good solid wires on them and the wires should be connected to the main aircraft ground bus bar or common grounding point. When airband radios are transmitting they draw more current than when receiving, so they need good solid power connections with plenty of current capability.