I am usually very careful about ripping and encoding my music. I used EAC to rip this particular album but on a different computer hastily configured. In one of the files the music slows down a little bit (like with a tape player when the tape has a problem (rolling out/stuck) the pitch of the music changes) for 0.3 second or so but it's noticeable. I don't have access to the original anymore. Could this be the original recording (actually it's a famous recording Beethoven: Sonatas Violin and Piano Sonatas by Oborin / Oistrakh) ? Because I don't think CD drive could produce such an artifact in the middle of the music. What do you think?

sounds like an analogue tape artifact to me, most likely the original.

Here's the part of the song. It occurs between 7th and 8th seconds in this file. What do you think?

IMO, it's almost certainly an analog artifact, like a "pull" or deformed area on the original master tape. You can slow down sound digitally, but your example does not demonstrate the typical side-effects of doing so (like extra noise, etc). I don't even know how a CD-ROM or ripping program could unintentionally do that...maybe by flagging a frame at an incorrect sampling rate? Plus, there are no noticable glitches between the problem area and the "normal material," which would be a pretty extraordinary feat for a CD drive to pull off with an error like that. Too bad you don't have the original for comparison.