Yes, My Cloud app for iOS backs up the complete Camera Roll to the My Cloud NAS; videos incl.
Also, to OP, you will get a few suggestions of how to use your iTunes music on the My Cloud. This is what I do.
I do NOT want my iTunes Media for the iTunes program administered anywhere other than on the same drive as my iTunes program, so I don’t make the NAS the location of where it is stored.
What I have done for years is to copy the iTunes Media folder to other locations such as external drives and currently to my MC NAS so I can access it with music apps and from other devices locally and remotely. Yes, I have to manually update the copied folder, but it does not take me long and it is what I prefer to do.
Various apps and devices can access the music as DLNA/UPnP or Samba shares. Music accessed as Shares is delivered in album track order, whereas music delivered via DLNA is delivered in alpha order; unless the DLNA app can be configured to arrange and deliver content in track order. The Android app BubbleUPnN can do this.
With iOS devices (which I have) you haven’t much choice as there are hardly any DLNA apps. Same may go for Sonos; it most likely receives the media in DLNA format only, meaning music is not in album/track order, but rather alpha order. This frustrates people that they cannot get the music in track order, so be prepared for that.
I keep no music or videos on my iOS devices; rather I stream it to my mobile devices from My Cloud, either locally or remotely. I also have an Android tablet where I can use Bubble and other Android apps. Also, I have a lot of Playlists, and DLNA is the only way to play from Playlists (if not actually using iTunes on device to play music). So. I rarely use an iOS device to play my music; instead I use devices that work via DLNA. So, to play my playlists from a playlist on tablet, and send music to devices (e.g. speaker) from tablet via Bluetooth. Or, play playlists from another device (e.g. a WDTV) into my TV and stereo amp.
So, to summarize. keep a copy of the iTunes Media folder on the My Cloud, manually update it, and understand that Sonos will access the music via DLNA, and this has its limitations, and advantages.