Hi Doc,
The stock mike is not all that sensitive, but I did have one thought on the matter. That 2600 is probably old enough to have some noise in one or more controls/switches.
If you compare the modulation level with the button out to how it sounds pushed in, you should see (hear) a clear difference. If not, maybe that switch has gotten "lazy" and isn't closing the circuit to provide full stock-level mike gain.
73