Sonos The Sonos gear was the first and easiest setup. Within the Sharing system preference I had to ensure that SMB file sharing was switched on. To do that, select File Sharing in the Sharing system preference, click the Options button, and enable Share Files And Folders Using SMB (Windows). I then mounted the NAS on my Mac Pro's desktop. With this done I launched the Sonos Desktop Controller application on my Mac, chose Music -> Set Up Music Library, clicked the Add button in the resulting Music Setup window, selected the NAS option, clicked Continue, entered a path to the NAS along with its username and password (so that Sonos would always have access to it), and clicked a couple of buttons to allow the job to be done. After several minutes, the audio files on the NAS were accessible to my Sonos gear.
iPhone, iPod touch, iPad Gaining access to the media on the NAS from an iOS device is straightforward provided that you've enabled Home Sharing on the device and one of your Macs is switched on (and also has Home Sharing enabled). Under such circumstances, you simply access the content by tapping More within the Music app, tapping Shared, waiting for the list of content to load, and tapping Play to play it. (Videos works similarly, though there's no More button. Just tap Shared, select the iTunes library you've shared from your Mac, and play the video.)
That's all well and good, but one of the reasons for storing media on an NAS is so you don't have to have your Mac running whenever you want to play that media. Fortunately, there are tools you can use to access that media directly.
Synology offers a number of apps that provide access to your media. Its free DS Audio app allows you to stream music from the NAS to your iOS device using an interface similar to Apple's Music app. With Synology's just-as-free DS File app, you can stream video files directly from the NAS to your iPhone, iPod touch, or iPad (you must enable WebDAV on the NAS for this to work). Makers of other NAS devices offer similar utilities.
Another option that I found helpful is Stratospherix's $4 FileBrowser app. It provides complete access to the contents of any networked computer, including Macs, Windows PCs, Linux boxes, and NAS drives. Just navigate to a folder that contains media, tap the file you want to play, and it plays using the iOS device's media player interface.