Sound cuts out

I have a problem with sound files in a PowerPoint 2003 presentation.
This was the first .ppt created by my daughter - I am trying to help her fix
this, but my PP knowledge is limited.

Speech was recorded as .wav files on a separate device and linked to the
..ppt.
The PP consists of about 20 slides, most of which have a brief speech
commentary (an individual wav file for each slide). Actual durations of files
range from about 2 to 20 seconds.

When first created, all sound files played properly when the presentation
was run. We now have a problem with some of the sound files. They only play
the first four seconds, then cut off.
As stated, it only occurs with certain files. Some files of 18 seconds
duration play OK. But if I take one example with an actual duration of about
eight seconds, it cuts out after four (and if I right-click the loudspeaker
icon and go to ‘Edit Sound Object’, the Sound options display shows ‘Total
playing time: 00:04’).

The files are all set to start ‘After Previous’. (I have checked that the
Sound Action is set to ‘Play’, not ‘Pause’ or ‘Stop’). There are no delays
set in the sound timings.
Each slide is set to advance both automatically and on click. We have
checked that the slide advance timings are much longer than the actual sound
file durations.

If we double-click the loudspeaker icon in the slide, the complete sound
file plays. If we run the presentation, or click the ‘Play’ button in the
Custom Animation pane, we only get the first four seconds.

Initially the PP was linked to the .wav files on the local computer.
Subsequently the links were removed, and the files embedded as per
instructions on this site. The problem seems to have arisen after this action.

We have also tried creating a new presentation using some of the same .wav
files (taking a couple of ‘existing good’ examples which worked OK), and some
new .wav files (freshly recorded, using the same device as before). These
files are embedded. The ‘existing good’ examples work OK in the new PP. The
new recording show the same problem of cut-out after four seconds.

All suggestions greatly appreciated!