Just posting a small success I had. Probably a lot of people already know about this, but i'm posting in case its useful to someone.
TL;DR Converting sound effects from 44.1Khz to 48khz results in noticeably better audio quality for all sound effects in my game on iOS (app built in XDK).
I was having a problem with my game audio on iOS (exporting for cordova and building in XDK).
When testing on an iPhone5 sound effects seemed to be low quality compared to the original .wavs. This problem wasn't occurring in Android where the sound effects sounded good quality just like the original .wavs.
In this thread, people are discussing a range of fixes for some iOS audio bugs that were occurring a while back (early 2016). I think the problems these guys are discussing have been sorted with recent releases of C2, iOS and Chrome, but there was a mention of how converting audio from 44.1Khz to 48Khz was a possible fix for problems they were experiencing at that time.
I wondered if this might also might improve my problem of generally shitty sound effects on iOS. And it did! The sound effects on iOS now sound as good as Andorid and the original .wavs.
Process was just to convert the sample rate from 44.1Khz to 48Khz (I used Audacity). Then export each sound effect as .m4a
Export from C2 for Cordova like usual.
Copy the 48Khz m4a files into the Cordova folder (www/media)
Then build in XDK like usual and test on iPhone.