Depending on your age you could get away with a lot lower than that and not miss a thing.
How would you use a low-pass filter on a newer LED TV with HDMI?
It would have to sit between your source(s) and TV and decrypt/recrypt the audio in realtime -- which means it would need to support the various codecs in use today.
Would be easier to buy external speakers and wire it up with those.
If your tv is the thing generating the inaudible signals, it could still choose to play them using its internal speakers, and send just the 'real' audio to the external ones.