Make sure the audio format is supported by IOS
Safari on the desktop supports any media the installed version of QuickTime can play. This includes media encoded using codecs QuickTime does not natively support, provided the codecs are installed on the user’s computer as QuickTime codec components.
Safari on iOS (including iPad) currently supports uncompressed WAV and AIF audio, MP3 audio, and AAC-LC or HE-AAC audio. HE-AAC is the preferred format.
Safari on iOS (including iPad) currently supports MPEG-4 video (Baseline profile) and QuickTime movies encoded with H.264 video (Baseline profile) and one of the supported audio types.
iPad and iPhone 3G and later support H.264 Baseline profile 3.1. Earlier versions of iPhone support H.264 Baseline profile 3.0.