I notice in "Volume mixer" volume bar for mozila (when i try listen to youtube) pops up and down but "Device speaker/headphone" on main system sound bar stays empty. Same for windows media player and everything. Thats with drivers from dell and drivers from autoupdate same issue.
I would suggest you to update the BIOS and audio driver on the system. Please enter your service tag # on the link below, select the OS, then download the BIOS from BIOS section and audio driver from audio section onto the system and install it.