$Name: 150 BT_up_1.wav, 0, 0.60, 2, 1500, 3000 ; 1 sec warm up
$Name: 151 BT_up_1.5.wav, 0, 0.60, 2, 1500, 3000 ; 1.5 sec warm up
$Name: 152 BT_up_2.5.wav, 0, 0.60, 2, 1500, 3000 ; 2.5 sec warm up
$Name: 153 BT_up_3.wav, 0, 0.60, 2, 1500, 3000 ; 3 sec warm up
$Name: 154 BT_up_3.5.wav, 0, 0.60, 2, 1500, 3000 ; 3.5 sec warm up
$Name: 155 BT_up_5.wav, 0, 0.60, 2, 1500, 3000 ; 5 sec warm up
Since all the beam warmup sounds should be heard at the same ranges, according to the original sounds.tbl, not hearing them seems to be a bug. I don't think I've ever heard the 3.5, 2.5, 1.5 or 1 second warmup sounds in-game.