Have you tried e-bay? I've seen people buy the speaker covers there. Maybe someone had that part as well.
How the hell did it crack. Just wondering so I don't do it.
EDIT - just looked on ebay and it appears that if you buy a new back cover the picture looks as if the part you need is included. I'd ask the seller first though.
Just search HTC One M8 parts.
Transformer Prime - Rooted Stock ROM.
Nook Color - Rooted running CM9
HTC Aria - RIP
Screen Smashed - I Loved that phone.
HTC Vivid - Backlight died and battery life all of 20 minutes - Retired
HTC One (M8) Current daily driver - Stock AT&T for now.