NAS drive shares (CIFS & NFS) are easily mounted and scanned by Volumio for audio content.
You can add USB hard drives, USB sticks, and use the free space on your SD card for extra or mobile storage.
There are lots of built-in internet web radio streams available, and you can also add your own.
Airplay connection could be a source for Apple devices
DLNA/UpnP is an out-of-the-box option to stream music in hi-res from an external source, using Volumio device as renderer.
ONLY FOR PREMIUM USERS:
You can add your own account of Tidal/Qobuz/Hires Audio streaming platforms
You can connect an external digital source via USB port (such digital turntable or CD drive)
You can stream a Bluetooth source
Finally many 3rd party plugins such Spotify, Soundcloud, Youtube and many more are available for free in PLUGIN section.