Please correct me if I am wrong but I believe this is the situation:
There is one set of discs produced in the USA/Canada (Region 1), one set of discs produced in the UK (Region 2), and one set of discs produced in Australia (Region 4).
* Seasons 1 and 5 in Region 1 have the music intact. Seasons 2-4 have substantial music substitutions including key songs. Some find this ruins the enjoyment of many episodes. Some episodes, such as Jimmy, have poor picture and sound quality. Seasons 2-5 are each on 3 double-sided discs.
* Seasons 1, 2, 5 (?) in Region 2 have the music intact. All 5 seasons are on individual discs.
* Seasons 1, 2, 3, 5 (?) in Region 4 have the music intact. All 5 seasons are on individual discs.
If you have a multi-region, multi-format (PAL/NTSC) DVD player and care about the music, then you might consider buying season 1 US, season 2 UK, season 3 Aus, season 5 US, and then wait and see on the long shot that season 4 is re-released with the music intact.
Note that PAL DVDs are recorded 4% slower to fit American 24fps film onto PAL's 25fps standard. This affects the audio and some people find the lowered semitone objectionable.
Note also that season 5 has not been released in the UK and Australia so the music may have been affected there.