[seamlisten] stop playpack at the end of non-gray segment

[Celebio]

We display a gray idea around the real audio segment to an idea of the context.

When we start the playback, it starts at the beginning of the non-gray area as expected. However it doesn't stop at the end of the non-gray area, it continues to the right gray area. It should play only the non-gray area except if the user explicitly wants to.

[Internal issue: 32]