I looked into backwards masking quite some time ago and checked the songs in question.
When I played "Another One Bites the Dust" backwards, I heard something that sorta sounded like "We like to smoke marijuana" but you could only hear it if you wanted to. It's pretty much nothing but a song playing backwards.
I played "Stairway to Heaven" backwards and found nothing.
However, many songs by "Electric Light Orchestra" have backwards masking. Some of the things I've heard:
"You're playing me backwards"
"Thank you for listening"
"The music is reversible, but time is not.... turn back.... turn back.... turn back"
Weird Al put a backwards message in his song "Nature Trail to Hell". It says "Satan eats cheese whiz"
The Beatles used backwards masking as well. Usually it was nothing but the same lyrics playing backwards. If you play "Free as a Bird" backwards, you'll hear John Lennon say "It turned out nice again"
However, the band Venom did a song called "In League with Satan" and you can clearly hear a bunch of "satanic" messages like "I'm gonna burn your flesh, I'm gonna taste your blood", etc.