Consumer audio devices do all kinds of psycho-acoustic adjustment based on the likely limitations of playback systems, likely music genres, and consumer expectations.
Headphones and small speakers are going to sound thin without them…i.e. a transparent system is going to try to reproduce sounds that most speakers can’t output.
None of which is to say your new system sounds good or bad. Just that what sounds good is subjective and context dependent.
that seems most likely to be the root cause of the change you're noticing.
if the source audio is 2-channel, do you have it only play on the L & R speakers, or is it getting output to all 6 speakers? that sort of upmixing needs careful tuning, if it's not done well it may sound "muddy" due to the slightly different delays in sound from each speaker reaching your ears.
if you want to isolate it as a controlled variable, hook up only your L & R speakers, play a test track, and see if the perceived quality degradation is still there.
This seems like a good candidate for an explanation. Stereo to 5.1 upmixing can be done on many different ways, and is likely to e different on different brands of sound cards.
Both in levels of sophistication but also just different options/choices, and one might have a preference for on or the other.
That smells more like a room problem. A slight change in frequency response will change the comb filtering and early reflections in the room and that will change what happens at any given listening position.
And all consumer facing audio messes around with frequency response.
To put it another way “sounding good” is a question on the audiophile spectrum where people demagnetize compact disks and can hear the difference of premium HDMI cables.
speakers 5.1 - what's in between the PC and the speakers? what amplifier? have you calibrated it (room size, speaker distances etc) ? where encoding/decoding happens? splitting sub to all-else? as sibling comment said, up-converting 2.0 to anything higher is not simple. Even to 2.1
youtube - the audio on youtube is.. not.. quality. i had a friend who used them as best-audio-compression-ever :) maybe, Opposite of all above, It is possible that now you are hearing their garbage that was unheardable before.
games - similar.. it's not an audiofile genre.
Get some proper sound out (e.g. flac or .ape or something) of something that you know how it sounds (piano? voice? some old electronic stuff?) hopeing it's not over-engineered at studio.. and check that. Better, multiple things ..
Mastering a recording for money?
Listening to MP3’s through headphones?
Or something in between?
Consumer audio devices do all kinds of psycho-acoustic adjustment based on the likely limitations of playback systems, likely music genres, and consumer expectations.
Headphones and small speakers are going to sound thin without them…i.e. a transparent system is going to try to reproduce sounds that most speakers can’t output.
None of which is to say your new system sounds good or bad. Just that what sounds good is subjective and context dependent.