try defining another handle and use that for second sound(or dont use handles at all.)
also you need to remove snd_remove(wavsound); lines.
I have tried it and it still doesnt work right 
var wavhandle;
var wavhandle2;
function gunsound2();
function gunsound2()
{
while(1)
{
if(mouse_left)
{
wavhandle = snd_play(explode_wav,50,2);
wait(-1);
// snd_remove(wavsound);
}
if(mouse_right)
{
wavhandle2 = snd_play(shoot_wav,50,-2);
wait(-0.1);
// snd_remove(wavsound);
}
wait(1);
}
}