Hi trinhlinh923,
Please try the following:
1. Update to the latest version of Little Musician (right now the latest version available is v1.5.810).
Click here for instructions on how to update LMs2. If you still can't hear the sounds completely or clearly after the update, use the Audio Diagnostic Tool.
Click here for instructions3. If the problem persists after doing all the steps above, please send a support request (at
http://support.brillkids.com/anonymous_requests/new ). Make sure you
attach your diagnostic report when you send us your request.
I hope this helps!