I don`t have the game myself and am not a programmer, But it maybe a sound priority thing. As the GP32 only has 4 sound channels (same as the Amiga), When you press and release the fire button the game may swap (quickly) between the bullet sound and the music (giving priority to both) and only allocates a single sound channel .When you press and hold the fire button, the game has to make the bullet sound repeatedly, So may use two or more sound channels to make the bullets sound faster, And so cutting off the music (lowering the volume). A lot of the amiga (and probably other systems) games did this because of the limited sound channels.
But i could just be talking bollocks.
Trooper