This sort of thing happens to me a lot, though actually what I get is a truncated sound, rather than no sound at all. Sometimes changing sounds helps, other times not (my sounds are ALL custom ones).
At first I thought I had corruption in my sound files, but deleting the copy on my phone & selecting a default sound, and then moving a new copy of the same file from my PC to my phone STILL doesn't help. All music or audio playback apps play the sound file fine -- even the preview of the sound will sound fine, but when the notification comes in, the sound gets cut off (its like the first second of it is just not there).
In truth, I've had issues galore with sounds for notifications on my 2014 PE Moto X -- it's the only phone I've owned that has ever had these sorts of issues. Stuff like volume too low, lag in responding to a change in volume, truncated notification sounds, sometimes NO notification sounds when volume is set to the right level, etc. I seriously thing its some sort of weird Moto or LP bug, as it'll happen on my husband's 2014 PE Moto X as well with his own custom sounds. Sometimes the sound works fine, other times not. I've never had another phone do this before, but my 2014 PE Moto X has been doing it off and on since I got it. I believe it got worse after the 5.1 update. I didn't spend enough time on Kit Kat on this phone to know if the issue was present there or not. If it were consistent, I might have ditched this phone for another by now, but most of the time for me, it's intermittent.