So this was new, in Premiere 2014, I usually change the speed on a lot of sound effects that I use for transitions. Most of the changes are slowing them down from 50% to 80%, whatever fits the duration or matches the music bed well. I recently cut a spot where I slowed a few sound effects to 10%. They sounded great. They even played fine after Premiere rendered the audio, but when I exported the finished spot to a ProRes file those effects where missing. There were other sound effects on the same track, some 100%, some 80%, and one at 50%, they all came through fine.
I ended up sending the original clips to Audition and doing the extreme slomo there and then back to Premiere, that worked. I'll have to do some experimenting, but there seems to be a threshold for speed changes on audio in Premiere.