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When the sound in a speaker sounds muffled or odd, it may be the impedance matching is off.
However, if you check the wiring, and if it worked fine previously, and the wiring is unchanged, then impedance matching is probably not the problem.
Try (as I said) checking the wiring, and reset the sound card in the computer, effectively, by doing a Shutdown on the computer, and leaving it shut down, for at least 10 minutes.
Start it up again, see if the speakers sound right again. Watch for the problem again even if it now seems cured... this may prove to be a problem that persists and might require the computer (and its speaker set) to be returned for an exchange for another (working!) computer.
Also check in Device Manager on the computer (if it is a Windows PC.) See if all is well with Sound and Audio devices.
And check in your Mixer or Volume Control set, to see if any speakers have changed modes, etc. If you have an elaborate Mixer Controls set, an 'audio effect' may have been enabled that switches into the odd mode you noted, doubtfully. Or the side speakers may have been put on Mute but not the subwoofer. (You Volume Control should show that, if so.)