I bought nothing but 13.2mm AAMG and ran a test game.... for Z fire they make normal MG sounds. Checking the unit and the weapon data does not show any assigned sound for that weapon so should be using the game default but when fired in the game they sound like a tank gun as you reported.
The weapon WH size is correct as that can influence game sound so it may be the only solution is to assign a sound but it's doubly strange (indeed!) that this is happening but also that it's not been reported before this
EDIT...... I assigned sound 165 to both the weapon and the unit weapon then re-ran my test game and now they sound like HMG.
This is certainly a weird one but a workaround is now in place and unless you are actively PBEMing you can change those in MOBHack and it will 'fix' your game in progress to sound less strange.
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Okay, will add the sound number you suggested. I was trying to figure out what was firing at my aircraft, missed the first text telling me then caught the second one and I am like odd but okay, wait no that's a heavy MG for the most part, not a cannon.