In reply to Co1in H:
One of the downsides of flat-screen TVs is that, being flat, there's no room for a decent speaker. So the sound tends to lack lower and mid response, and ends up pretty tinny. Which I guess is one reason for the profusion of 'sound bars'.
I opened my TV up, removed the piddly little speakers, and fitted 3.5mm jack sockets so I could plug in some cheap speakers from a second-hand mini hifi. In the end, I found a Creative surround sound system, and use that, with a feed taken from the headphone socket. I guess I could do it 'properly' and take the digital audio feed into a decoder and amp, but ICBA...
My local charity shops often have computer surround sound systems for about £10. The speakers in those are likely to be better than the feeble things in the TV.
As for the smart TV sounding better, it may just be that, being a smart TV, they've spent a bit more on the audio chain (or audio signal processing), or, if it's a bigger TV, the speakers are also a bit bigger. Or it's just louder...