You would have to technically explain how a computer - old one vs. new one - makes music sound better?
If your music resides on a hard drive and the computer is merely passing on the X's an O's to a USB cable (which I have found does make a difference) then I am stumped at how that would be of importance.
The only technical aspect I am aware of that may come from the computer is jitter. However, most DAC's nowadays incorporate jitter reducing remedies so that is no longer an issue. It did give rise to the "jitterbug" for a number of years.
I have read that some here state the software they use (typically Roon or jRiver) has an effect on sound, but I assume that is when streaming or accessing music over a network. Using Windows Media or iTunes to play music that resides on the local computer hard drive should have no effect on sound.
I have used a few computers and there is no difference in sound. If anyone asked me I would tell them an old computer is just as good as a new computer when playing local music files. Dedicating a computer (Jukebox computer) is recommended as I could see a whole host of issues if one is playing music on the same computer they utilize routinely.