Inspiron 660, Windows 10, Bad audio quality

Hallo,

during the last weeks the audio quality output of my Inspiron 660 has got worse.

Either when I use the headphones or a speaker, both front panel or rear panel connected, it happens that the sound distorts, or switch mono to stereo and vice-versa, or it happens also that the volume go up and down or that. That  happens  with any kind of source (radio, youtube, audiofiles, spotify,...) and if I listen to the same source with my tablet or smartphone, sound is ok.

Drivers are updated.

I'm not an expert of pc stuffs, I'm just thinking to buy a new sound card.

What do you think, can someone help me?

Thank you

E.