Yeah I would try headphones see if has same sound, could even be the powered amp/woofer at fault, try moving speakers around as maybe one output to speakers, another speaker set, even if latest drivers could be corrupted so download them again from Dell with service tag & install again, restart see if has any effect, Realtek also has them on there site but you'd need to know your audio chipset so dont bother, the virtual room surround effect settings could create that effect like Hall, Stadium etc, so check the Realtek speakers settings as I'm leaning towards this, onboard audio doesnt go wrong very often TBH so I'm not convinced its hardware faulty but try all the above.