Now that I'm finally getting longer flights on my foil in light wind, I'm starting to clearly hear the different beeps from the GT31 as I sail along. It's always been enabled but I guess when you're "slapping" in the chop there's too much background noise compared to foiling.
im sure it's trying to tell me speed or something, but does anyone know how to decode it? Is it Morse code or something else?
Your are right James. It is genuine audio feedback, but most people don,t hear it so they don't notice it or know it is there.
The specs are available somewhere in the GT-31 manual, but as I remember it, the GPS beeps on a run while your average speed is increasing, and stops beeping when your average has peaked. The light on the face blinks in synch with the audio, which is probably more useful for most sailors.
At the end of a run, the GPS will beep out your top speed in a kind of code. e.g.: three short beeps = 30 knots, followed by a combination of short and long beeps to tell you the second number e.g.: three short beeps folowed by a slight pause and another three short beeps in 33 knots. I can't remember the exact code but I am sure someone else does.
I used to hear it all the time because I had my GT-31's in my helmet. The end run beeps were quite clear. But I had to remember to turn it off one one of them or it was just a confusing jumble of beeps. 
It can be quite useful for speed runs. I dont use the GT's as much now and I have the visual feedback from the GW-60 and the audio spoken feeback from GPS-Logit.
I found this using the Internet Archive:
Speed Genie Sound: GT31 Speed Genie provides bio-feedback sound/light indicators: acceleration, max n-second average speed increase and also audio-announces the max avg speed result. Sounds are designed for GT31 mounted inside the helmet to provide sailors audio-feedback so that they can focus their eyes on sailing to improve safety and maximize speeds. LED bio-feedback is designed for arm-mounted GT31. Set (ALERT>SPEED GENIE>ON) to enable Speed Genie helmet bio-feedback. If you need sound, set (ALERT>BUZZER>ON). The bio-feedback is delayed about ~2 seconds, due to the limited bandwidth of the GT31 Doppler system. Please see README file for details.
Then I looked up an old GT-31 firmware 'read me' file and found this:
=== V1.2(B1218T)=============================================================
- Added Speed Genie sound at [ALERT]/[SPEED GENIE]
- Supported Speed Genie bio-feedback sound indicators: acceleration, avg speed and speed result (Tom Chalko)
A. When Speed Genie speed threshold is exceeded:
1: Short Buzzer Sound when MAX N-second AVERAGE grows
2: Long Buzzer Sound when speed grows (acceleration indicator for high speeds)
3: combined Long Buz/Short buzz sound, when 1 and 2 are true
B. During Speed Genie result display
1: Festive sound pattern if the Personal Best average speed was achieved.
2: 3 digits of the N-second max average speed result encoded in the sound as follows (code similar to Roman numerals, except for 4 and 8)
1 = .
2 = ..
3 = ...
4 = ....
5 = _
6 = _.
7 = _..
8 = .._
9 = ._
0 = __
where
. = ~100ms short sound
_ = ~300ms long sound
__= ~600msec for zero (not truncated)
silence separator between digits: ~300msec silence
The numeric speed result is repeated 3 times and then every 2 minutes when Speed Genie results are displayed on the screen
