Try changing the behaviour by instead attaching the audio to the On Success caption of the button object. Set the caption to 0% Alpha so that it is effectively invisible at runtime. (Remove the advanced action > play audio from the button.)
Test this way of playing audio to see if it performs any better at runtime in HTML5.
Be aware also that in HTML5 content, especially in iPads, there appears to be very little preloading of the assets. So you don't get quite the immediate response that you may have been accustomed to in HTM/SWF output. What seems to happen is that when you click something in HTML5 the assets required by that action are only then requested from the server. So there is often a delay while waiting for the asset (in your case a sound file) to download and start playing. User's that are trying to play video in HTML5 output often report similar annoying delays here on the forum.
Whether this behaviour has been improved or not in Cp8 I cannot say. Perhaps others that are using Cp8 can chime in with their experiences.