Previous slide while music playing (solved)

[Pansmanser]

In an Impress presentation I have a sound file which is activated by a slide transition and plays across several slides. It works fine, playing through several slides with a ‘no sound’ transition, until it reaches a slide with the transition ‘stop previous sound’ and it stops. So far so good.
However, if while the sound is playing I try to move to the previous slide (page up, left arrow, backspace etc) the program advances to the next slide instead. After the ‘stop previous sound’, the slide changes revert to the expected behaviour. Although this seems quite clever (to know that a ‘no sound’ transition is irreversible while a sound is playing), it is sometimes annoying.
Can I do anything to ensure that ‘previous’ means previous?

LO 5.1, Ubuntu 14.04 (I believe this also happens on Windows 7 and LO 4.)

[Pansmanser]

Further examination of this issue shows that the problem is incorrectly described above,for which I apologise. The problem is that when an automatic timing is set on a slide, the slide show cannot be reversed. This question should be closed rather than solved, if I was able to do so, and I’ve posted a new question here.

"previous slide" with automatic advance

When an timed automatic advance is set for a slide in an impress slideshow, “page down” overrides the automatic time, but “page up” does not. In other words I can advance the slide manually, but not move to the previous slide. It is not possible to “reverse” the slideshow from a timed slide other than by “alt-page up”. When using a remote ‘clicker’, alt-page up is not available. Can I do anything about this?