a Soundbed is normally used behind the voice over and is normally a instrumental track. it has to be appropriate to the documentary.
the voice over is used to introduce the documentary and its topic. it is normally spoke in a clear, informal and relaxed voice so that it fits with he style of the trailer for the documentary.
the script for the voice over tends to use rhetorical questions as this will draw listeners in to listen out for more information about the programme.
SFX can be used to add appeal to the radio trailer and the sound effects that are used will be relevantto the trailers themes and topic.