I think everybody is right here. One possible reason you don't see studs is that there might be a shear panel behind the drywall, just because it might have been a cheap way to span the space, but I don't think that is legal, and I don't think anyone who wants to build a fire inside a house, and have insurance, would mess with a fireplace that is not built legally. There IS wood there!

Don't worry about the actual location of the studs. I have mounted several plasmas where the mount's slots did not line up with the studs. It is truly bizarre how so many of these mounts are made with specific slots that force you to place the plasma somewhere; it just ain't the case that the studs in everybody's houses are where the mount manufacturers want them to be!

If no studs line up with the mounting holes, then use some hefty wall anchors for shear; that is, use toggle bolts or something in the mount's holes to hold the mount level and to position it at the right height. Then (measure and plan for this, of course) drill AT LEAST two holes, preferably more, in the back plate of the mount so you can put a couple of lag bolts directly into the studs...you will have to drill these holes yourself if the mount's slots don't line up with the wood.

Those last two bolts actually bear the weight, while the toggle bolts position the mount for you to install those bolts. The toggles add to the strength, but the bolts do most of the heavy holding. Reminds me of a place I used to work where they made school record players; in the wood shop, they shot some thirty staples into the edges of the wood pieces that made up the record player's box. But the staples were not for the purpose of holding the box together. They were for holding the box edges tightly together long enough for the glue to dry, and the glue provided the strength.

Exactly what we do...