Hi skak
While you CAN rip and play it is BEST not to. The Brennan units only have a very small computer in them and they do not like doing two things at once.
Best advice is to set yourself up to rip your CD collection , about 20-30 CDs at a time then leave the unit on overnight to allow it to compress these CDs to FLAC then next day do the next batch of CDs.
It take a bit of time to get through your collection but it is a one time job!
Also every 3 days do an incremental backup of your unit ("Export to USB A or C") so that you have a good backup of your music collection.
Hope this helps
Fred