The recommended time is at the start of layout/ upon layout loading. The only problem that can really arise is if you are preloading lots of audio at the same time as this can slow down browsers, so the recommendation is no more than 2 or 3 audio objects being preloaded at once.
If you find that the preload is still causing a delay, this is because you are preloading a large file. It is best to keep music in the music folder so that it is streamed and have sounds in the sounds folder, which are downloaded before being played. Preload doesn't apply to objects in the music folder.