This is an annoying sound bug. The notification tones in messaging/email apps stutter; meaning there is a break while playing the tone. Either there is an annoying 'click' at the beginning, which is mostly the case, or the tone 'breaks and resumes, stutters' sometimes in the middle while playing.
I've tried disabling the sound enhancements and different combinations too, but it happens almost all the time during every notification; SMS, Whatsapp, etc. I am on .757. Was happening on .681 too. Not that it is a major issue, but it is quite annoying.
There is one thread on this forum like this some months old, but without a solution. Anybody aware of this problem?