I've Exported an Avid MXF-OP1a file in Avid Media Composer 8.4.4.  There is no audio from the exported tracks 3 and 4 which is music. I can hear tracks 1 and 2 from my sequence which is nat. sound and soundbites in the exported file.  Any help appreciated.

Use 50Mbits setting, if available, not 35Mbits. 

You need to do a mixdown first, if you want a mixed export otherwise in that version it is exporting as direct out.  So if you have 4 tracks selected its exporting 4 seperate audio tracks.

Thank you, the video and audio mix down worked for me.