When I hear a song that immediately follows another one - like a DJ mix, the radio, or even on an album where the gap is very short - would it sound wrong if the 2 songs were slightly out of tune with each other?
I can see two possibilities:
1) Songs are always out of tune with each other and we are just very good at retuning our ears to the new one
- or -
2) Musicians are very good at making sure their recordings are always perfectly tuned to a standard (rather than just to each other) and it would sound very odd indeed if I ever heard a segue that was out of tune.
I genuinely have no idea which of these is the case - if any!
EDIT: In response to a comment.
By tuning I mean the tuning of the instruments used in the recording - and hence the tuning that the vocalist will automatically adjust to.
For example, without using a tuning reference, I can tune a guitar to itself, making all 6 strings sound good together, whilst not necessarily being accurately tuned in a technical sense, where A is 440Hz.
If a bass player joins me, he can tune his bass slightly to match me; neither of us are "right", but no-one would know.
If a pianist joins us, that's different prospect; retuning his instrument is a massive undertaking - so we would retune the guitar and bass to match the piano.
My guess is that more recent music will be more likely to be tuned accurately due to the technology being used, but we still regularly hear music from the 80's and earlier - most of which could have been recorded "out of tune".