Inspiron 24 5240 AIO

I have upgraded from an Inspiron 660 which had external speakers to an Insprion 24 AIO with built-in speakers I was expecting the audio recordings to be at least of equal quality to the input whether that be digital broadcast or external CD. However, when using Stereo Mix as an input device, so I can record sound from the built-in speakers, the lower frequencies of the recorded output are distorted i.e. it's recording sounds that are not discernible in the source. I'm using WavePad as a recording device but get the same distortion when using Audacity. Having verified that the best drivers for my device are already installed, Dell Technical Support advice says the problem, falls outside its scope of support. What advice can the user community offer that will resolve my problem?