It was not unusual for mainstream music to recorded at varying speeds, A good example of this is "Kind Of Blue" by Miles Davis. This was not released at the correct speed until the 1990s. You can usually tell when you try to play a tuned instrument along with the recording and you notice that there is a slight pitch variance of up to a quarter tone or so usually due to minor speed variations on primary recording equipment and secondary transfer equipment and mastering. In the case of Miles there were three tape recorders in use. Here are a couple of discussions of other examples Billy Joel and even a classical record! Symphony No. 8 in C minor, opus 65