iPhone and iPad don't play audio for certain videos Here's the metadata from a video file that does not work on iPad and iPhone. Does anything about the audio stick out at you?

avprobe version 0.8.17-4:0.8.17-0ubuntu0.12.04.1, Copyright (c) 2007-2014 the Libav developers
  built on Mar 16 2015 13:26:50 with gcc 4.6.3
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Introduction.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf54.2.100
  Duration: 00:07:54.14, start: 0.000000, bitrate: 1148 kb/s
    Stream #0.0(eng): Video: h264 (High), yuv420p, 1280x720 [PAR 1:1 DAR 16:9], 924 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc
    Stream #0.1(eng): Audio: mp3, 44100 Hz, 2 channels, s16, 216 kb/s
Not at all - nothing in that metadata looks like it would be problematic. 


MP3 support is quite good
h264 is one of the preferred codecs for iOS since quite some time. 


You may have to check the correctness of the file or transcode it or get a more thorough error confpdition if ones like this fail to play on iOS.