I'd disagree with DumDum above (sorry buddy) as when I swapped out the stock speakers for some £18 (yes, that is one eight) JVC speakers from eBay (designed to replace OEM) it was miles better. Its no audiophiles dream but better bass, broader frequency and better clarity. Upgrading the headunit to a 2nd hand £10 Sony unit and adding a pair of the same £18 JVC speakers in the rear (mounting points and cables are there) massively improved it to the point its reasonably bearable now. Total cost to me was £18 x 2, £10 for headunit, £5 for speaker wire adaptors, £10 for headunit surround.
Reducing vibrations also helps and it doesnt help that Renault mount speakers by 2 screws instead of 4.