The Heavy Metal FAKK2 theme is called Heavy Metal Fire. It is an [bad word] by Zak Belica, with the vocals being performed by Jenny Lougheed.

I've been naughty lately 
All kinds of thoughts in my head 
It seems that I have a notion 
I just wanna see red 

It's time so show 'em the heavy metal way 
I'm gonna kick your [bad word] five ways to Friday
You cross me now and I'll cut you in two -
Feel the sword - it was good for me too 
'cause 

FAKK2 [bad word]  at you 
Heavy metal fire 
Is my desire 

Here we are now 
Locked at each other's throats 
For all time 
Killer and savior 
Killer and savior 

Who will win now 
I think you know how the story goes 
- evil defeated 
FAKK2 is Victorious 
Victorious 
Victorious 

It's time so show 'em the heavy metal way 
I'm gonna kick your [bad word] five ways to friday 
You cross me now and I'll cut you in two 
Feel the sword - it was good for me too 

FAKK2 [bad word]  at you 
Heavy metal fire 
Is my desire 

Here we are now 
Locked at each other's throats 
For all time 
Killer and savior 
Killer and savior 

Heavy Metal 
FAKK2 
I'll be waiting 
For You