Why we can't hear when we yawn?

There are two answers.
If you mean you can't hear yourself yawn, then it could mean you aren't yawning loud enough to hear it or you can't hear very well.

If you mean you can't hear anything when you yawn, then that could be a problem or a sign of poor hearing. Everybody is different but when I yawn, I can hear stuff around me. Good luck and I Hope this helps =]