I've converted a live snare track to midi using the new drag/drop method so I can get a better sample but I now can't use my audio plugins. How can I convert it back to audio so I can use the PX Percussion strip??
 
Thank you.

You can put any FX in the FX bin on the audio or Simple Instrument track that's receiving the output of whatever drum synth you're using to replace the snare.

Insert whichever plugin is going to generate your new drum sound and point your newly created midi track to it.

Thanks guys.  I think there must be a difference between "simple instrument track" and "midi source" as I'm sure it's worked as you said with my other projects. Still much for me to learn...

Yes, a MIDI Source track is the track that drives the soft synth, playing back the notes you extracted from your original drum audio. In order to hear that synth's output (and apply FX to it) you need to have an Audio track with that synth assigned as Input. A Simple Instrument track combines the functions of a MIDI and Audio track, and the FX bin you see in the track header is for the audio portion. The 'FX' bin on a MIDI track is for so-called MIDI FX that process MIDI data as opposed to audio data.
 
I hope that helps clarify things. I recommend using separate MIDI and Audio tracks until you thoroughly understand the 'ins and outs' of both before using Simple Instrument (SI) tracks. I you've already created an SI track, you can right-click it, and choose Split Instrument Track to separate the MIDI and Audio parts.
 
Once you really get what's going on with the separate tracks, you can re-combine them and click the MIDI and Audio tabs at the bottom of the Track Inspector when the SI track is selected to get access to the independent controls (and FX bins) of each as needed.