Thunderbolt display sound problem

Hello community, I have a strange problem with my thunderbolt display sound.

The hook up is: iMac 21.5 late 2013 and Apple 27" thunderbolt display. Everything on thunderbolt display works fine, camera, USB, display etc. are all normal.

But when playing sound through thunderbolt display (either youtube from web or music app), speakers work fine, but when I open a new program (word, excel, etc) or open a new tab (safari, firefox), sound will stop for 1 or 2 seconds and comes back normally. Playing sound through iMac does not have this problem.

Anybody have same problem? Any solutions/suggestions? Thanks.