Further testing reveals that if you delete the media filenames from the list in the offline.appcache everything works as it should. If you leave those in the list then the iOS web app will not play the sounds anytime after the first loading and the sounds will not play if you reload in iOS Safari. Something is broken with caching the sound files. I don't if it is Construct or iOS but it is definitely broken.
Strangely if you preview on LAN using the web server built into Construct everything works as it should. If you save this version to the home screen as a web app it works but continually downloads data from the server. What causes this when using preview on LAN? Why is the web app continually hitting the Construct web server?
I have also found that it is very easy to completely freeze iOS if you close an iOS web app and try to re-open it immediately. Often it requires a forced hard power down of the iOS device. Safari on iOS appears to break easily and crash the whole OS.