Is there a difference in sound quality between 720p and 1080p?

Not from what I can tell. It depends more on the speakers and headphones I think. 720 and 1080p are pretty close so it's hard to tell. But you can hear the difference when you listen on 240 or 360p

http://youtube.com/user/chaoticmatters

And what people are saying about it being pixels are correct but youtube is different. The audio quality does change as you change the video quality.

the hdtv will upscale the 480p sign to the two 720p or 1080p. you will no longer observe the type in image from a 480p sign. those appear like 40 or 40 six" hdtvs, you will no longer see a difference between 720p or 1080p. in case you pick 1080p, get a minimum of a 50" television, the bigger the extra desirable.

The p refers to pixels, and the larger the number indicates more pixels vertical and horizontal. This applies only to the visual quality, not the audio quality.

No difference. Those refer to the horizontal video resolution and interlace type.

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P stands for progressive, as in all the lines are drawn at once, as opposed to i, for interleaved, where every other line is drawn.

theoretically no but a more expensive 1080 TV is likely to have better audio equipment and speakers fitted