Sound suddenly muffled

Hey guys,

I have a problem with the speakers on my iMac.

The audio was completely fine till yesterday (I'm not sure when exactly), out of nowhere, it started sounding very muffled. I don't know how else to describe it, it just sounds very, very bad (but it sounds ok through headphones).

I have checked the preferences and everything looks good, I restarted it but nothing really has seemed to work.

It seems to be a hardware problem, are the speakers blown? Or can it be some wire not connecting right?

(The iMac is a 27" Retina 5K late-2015 with macOS Sierra, if it helps at all)

Thanks in advance

Inês