Is the audio coming early or coming late?
The delay is probably coming from one of two things: your TV's internal 24p processing or your USB DAC (or maybe both!). Should be easy to determine if it's just one or the other. Watch your videos at 60hz refresh rate. If you notice the same kind of sync issues that happen when you are using 24hz refresh rate, than it's not your TV adding the delay. If you DO notice that the sync is much better on 60hz, then it's the TV...
For some reason 24hz refresh rates seem to add 5 frames of delay on a lot of TVs. It did on mine. A bit of simple math will get you the appropriate length to delay the audio to compensate:
1 / 24 = 0.04167 seconds per frame = 41.67 ms per frame
5 x 41.67 = 208.35 ms
See if adding a 208ms delay to the audio helps.
If it's the USB DAC causing the delay, it's going to be a little more difficult. Instead of delaying the audio, you'll want to play the audio early. It'll take trial and error to find the right value, but you'll want to use a negative number for the audio delay, "negative delay" if you will