serial port data loss in windows 7 ,when sound is played

Not believable; not enough information.

Obviously, it's impossible to give any answer due to lack of important information. Just some advice:

Check up your observations -- they can be wrong. For a human being, it's typical to mistakenly recognize a random coincidence as a correlation, a trait fixed by the evolution.

If, by a rare chance, there is a correlation, it should not do anything with the nature of "distracting action": it could be sound or something else. To analyze such possibility, find out if there is race condition in your code:
http://en.wikipedia.org/wiki/Race_condition

If you still need help, you need to reproduce this problem on a very simple but complete code sample (not your full code). It already can help you the solution. If not, post the sample, using "Improve question".

--SA