I don't have the Garmin set up that you have, instead mine is an Icom 504 with remote speaker mike in the cockpit, but had the same symptoms of none or very little sound coming from the speaker part of the mike. I sent it back to Icom describing the issue and they fixed it by replacing some internal part.
Other than that it's possible for the pins and sockets in the plugs to get dirty or corrode, so keeping them clean and as well protected from from the weather as is possible, is important. The external socket on the binnacle on my boat has a cover which is always replaced whenever the mike is removed. In addition I have a canvas cover which goes right over the binnacle and down to deck level which also helps to keep it and the instruments dry and well protected.