I dunno what's been said in this thread already, but I've listened to AYNIL millions of times and seen the videos thousands of times, and I always thought for sure it was Paul... It sounds just like Paul's voice with his little tones and accents and all that...
However, there is a bit of mystery after carefully listening to the voices in the song near this point at the end...
Here's a little rundown:
1. At approximately 2:30 a voice (Paul's) shouts "all together now"
2. Then (the same voice), "everybody" at 2:35
-In the video, you can cleary see that Paul is saying this... And his distance from the mic was about a foot or so further back than he was for the rest of the song, which resulted in the change in the sound of his voice
3. At 3:06 Paul "yee-hoo"'s
4. This is followed by John's "Ye-hay!" at 3:08
5. Between 3:10 and 3:17 John begins with "y'need", then repeats the chorus again, then ad-libs a bit. After this he continues on with "love is all you need."
6. At 3:21 a voice that sounds identical to the one in 1 and 2 of this rundown shouts "Oh yeah!"
7. At 3:23 the voice continues with "love you yeah yeah yeah"
8. At this exact time, Lennon's voice stops singing the main chorus
9. The first voice continue's with "she love you yeah yeah yeah"
Listening to several bootleg versions didn't help... So I'm still under the influence that it's Paul, because his voice sounds so much like that...
Also, later in the song on bootleg versions (where it doesn't fade out), you can barely hear John saying stuff as the song continues, which don't sound anything like the voice singing "she loves you", etc...
Anyway, if there was a check mark to check when a post a utterly useless, my post would be checked... Sorry all