Found out the problem and fix for ASX and CorePlayer. Background
1. MC currently stuff MP3 into an ASX Container so it will not play in Windows Media Player (you can only have WMA/WMA etc in a ASX container), hence
2. Changed the file association for ASX to Coreplayer but the paramater it was passing was %1 not "%1" now it plays fine (coreplayer was complaining it could not find the ASX file itself)
Couple of comments in the WinMo space.
1) All WinMo devices have Windows Media Player and as we know it will only except streaming playlists for WMA/WMA etc in the ASX container and this is hence the ideal out of the box solution, however;
2) If we are not able to transcode to WMA elegantly on the fly and instead have to rely on transcode to MP3 for Webplay then there is no need to ASX containers at all - please just use m3u as the default for ASX is WMP and it will fail.
As it stands currently, WinMo users then need to get a new player, and then work out how to (correctly) , manually assign the file associate ASX to this player (needs another 3rd party program like Total Commander.
Thanks
Nathan