Hi
Hmmmm; tricky!
I've worked on a few Linn Kivor units (they've now been obsolete for a good few years), which were of a similar 'concept' to the iMerge, but likely worked in a different way. One if my friends has worked on iMerge, so I'll ask him if he can give me any info about them as I've never worked on one, myself.
The Kivor was - at the time - a very upmarket and nice sounding system, which worked by ripping CD's to WAV files, then obtaining meta form somewhere (maybe also Gracenotes) and building up a database of metadata (it contained a bunch of nice DAC's and a bunch of RCA sockets to feed analogue signals to various rooms). When extracting music from a Kivor (to change to a NAS and media player setup) I tended to FTP all the wav files and database to a NAS, then use a script (supplied by Linn) to convert the WAV files to FLAC files, then it tagged them by using the information from the kivor's database, thus ending up with a tagged FLAC collection as opposed to a WAV and databases proprietary system.
That you can back up your iMerge to a NAS and that the Sonos can see the files (and read the metadata) implies to me that it might instead just be ripping them to mp3 files (as opposed to WAV files). Either that, or it's doing something similar to a Kivor and the backup is using a script to convert WAV files to FLAC files and tag them in the same way that I described for the Linn script (but I'd highly doubt that). I thus suspect it's actually the former, and thus it's just ripping them disks to lossy mp3 format and tagging them at the time of the rip/encoding process, and your backup is simply a copy of the mp3 files as they exist in the iMerge.
Assuming my assumptions are correct, the most obvious way would seem to be to carry on with your current backup strategy (which is sound), but instead of mapping the Sonos to the NAS share, map it to the iMerge share and it will index the mp3 files on the iMerge. So, when you rip a new disk, it will compress it to mp3 format in the iMerge (as usual) and then you simply update the music index in the Sonos (rescan the library) such that you can play it a few minutes later.
Of course, that depends on my assumption being correct about the iMerge ripping/encoding to mp3 (which format of files do you see in your backup?) and it does not address the art issue. Mapping to the iMerge share also assumes the iMerge even provides a network share (it might not) or there is an option to turn that feature on (or hack it on via the command line; assuming they even let you have SSH access to it).
Ultimately, the better solution might be to decide on which system you wish to use and sticking with it. If you now have a few Sonos units in the house, and if the iMerge does encode to mp3, it might be worth re-ripping your entire collection to FLAC files as that wll give you a sound quality enhancement to the Sonos rooms. To be honest, it won't be a huge difference via a Sonos, but it is audibly better. In addition, for your main system (say the Sonos connected to the HiFi in your lounge) you can add an external DAC to that Sonos and you'll find (assuming you get a half decent DAC) that it makes quite a difference to the sound. That certainly would let you hear now much better the FLAC files sounded and if that's a potential option for your future system, it makes the re-ripping option even more attractive (and, of course, it solves the missing artwork problem).
Sorry, without knowing the iMerge, I am making lots of assumptions. What I can say is that a Sonos with a decent DAC sounds not too far behind something like a Linn Majik DSI (which was a £2500 media player) so it is worth doing if sound quality is high up your list for the main room. Of course, it means you'd then have to rip CDs for the NAS/Sonos system and again for the iMerge system, but to be honest, I'd be tempted to cut mt ties to the iMerge and migrate to NAS/Sonos (and back up a NAS FLAC collection).
How you move next depends on what your priorities are, what you currently feed from the iMerge (ie how important it is in your system hierarchy), how many Sonos units you have (is that the main source of your music), etc, etc. Maybe first do some research to find out if the iMerge can be accessed (and it's contents be made available as a network share) as mapping the Sonos's to that would solve the new rip problem, right away.
Bri
NB That the iMerge backup takes an overnight run to complete indicates to me that it's not an incremental backup (eg rsync) or it would only take a few minutes or backup the changes (new album) rather than overwrite the entire backup. That means there's little point in using an automated artwork grabber to add a jpg to each album on the NAS backup, as the nightly iMerge backup run would just overwrite them with art-free copies of what's on the iMerge, so that's another reason why I'd question persisting with the iMerge as opposed to just moving to a NAS and Sonos system (over and above the sound quality improvements attainable by moving to a FLAC collection).
