How to mount TV to basement wall under stairs?
As you can see there was apparently already a TV mounted there beforehand. 



The person who installed the dual speaker assembly cut off one of the studs and this could be of some concern. You could try to anchor that stud to the side of the stair support to make the stud more secure. Alternatively you could screw a flat 2x4 down (shown as red outline) on top of the lower plate just under the speaker assembly. Then a second flat piece of 2x4 depicted in the yellow outline could be screwed to the added red piece and into the stub of the stud above. The stud may need to be shimmed out with an appropriate piece of wood cut to size. From your pictures this looks like it would clear the rear end of that one speaker. Do use screws to fasten the pieces as the pounding of nails with a hammer would likely blister the nails/screws that hold the drywall in place. Since the added 2x4 pieces are relatively small it would also be advisable to use some construction adhesive to glue the pieces together before screwing. Once set the glue will make the assembly much more solid. 



So as long as you put the sturdy mounting screws for the TV bracket into the studs in a proper manner (i.e. predrill with the correct diameter pilot drill) to prevent splitting of the studs you should be good to go.