Welcome to the forums!
If you haven't seen it, some GL504GM customers seem to be discussing a similar issue at the thread below, but not sure how much it can help.
https://rog.asus.com/forum/showthread.php?107024-GL504GM-built-in-speaker-volume-fluctuating-loud-so...If you are interested though, new bios versions have been released yesterday (the ASUS download site will include the date when the files were posted) for both the GL504GM and GL504GS that we are hoping can help with this, but I always prefer to wait for customer feedback to be sure, so basically, no guarantee that it can help.
If you can live with the issue a bit longer, you may want to wait to see if others can provide more feedback about those bios versions since it's also possible there could be some bugs if you're a worrier like me or don't want to take any chances.
Anyway, just something to consider or keep an eye on and below is the link for the ASUS U.S. GL504GS download page.
https://www.asus.com/us/supportonly/GL504GS/HelpDesk_Download/