Web video sound problem

Hi all,

I have recently completed a video project for a client using Final Cut Express (exporting in h.264 and sound AAC) before uploading to YouTube.

However, when the client watches the video in their own office or the video is played on an iPhone through the phone's built in speakers, it only plays the music and not the overlaying voices. However, when external speakers are plugged into the iPhone/their PC, everything is audible. The same problem occurs when I try using Vimeo.

Also worth pointing out that it works fine on our office PCs, Macs and iPad.

I've checked all of the settings and they are consistent with every other web video we have produced without a problem, and the equipment we used was no different either.

We're completely baffled and have a frustrated client on our case! Can anybody shed any light on what's going on?

Thanks