This is a known problem but I couldn't find an acceptable workaround. 

Do you know why webkitAudioContext can't decode the audio files when they are base 64 encoded? I haven't done any more tests on this recently, so don't know if anything has changed since moving to WKWebView and if this works now.

It is not about decoding base 64 files. This example http://html5doctor.com/taking-web-audio-offline-in-ios-6-safari/ works and it is base 64 encoding.
I think it might be caused by something like this: http://stackoverflow.com/questions/10365335/decodeaudiodata-returning-a-null-error (the problem could be due to the mp3 file itself) but the fix proposed in this post does not work.

Thanks, yes the decoding seems to fail, I'll have a look at this again, trying alternate MP3 files. 

This is happening because the loadSounds function is wrapped in a "if (!IS_IOS)" condition.  Removing this throws an exception, however, as AudioContext() cannot be found.  It looks like iOS requires using the "webkit" prefix for this class, so I did a quick attempt at enabling that code on iOS and initializing the audio context using "new webkitAudioContext".  With this change, no exceptions are thrown (i.e. iOS found webkitAudioContext), but the sounds are still not playing...

Yes, I mentioned that in #0.
The error comes from this.audioContext.decodeAudioData(buffer) (using webkitAudioContext). The error returned by this function (by adding a 3 argument) is null.

re #5: This was done because there wasn't a suitable work around at the time. Am looking at generating the sound files a different way.