This is very, very weird. I don't think I will ever play this song at
Christmas for my nieces and nephews again! I think they call this kind of thing
'backward maskings' or hidden lyrics.
Hopefully your computer speakers are very clear as some of the hidden
words are hard to make out.
Supposedly just about any version of 'Jingle Bells' played in reverse
will have the same hidden words in it, so if you still have it on a
record & record player still works, give it a try.
Whatever you do, don't play this in front of your kids, this is very