ACX wroteHi njp,
username and password inputs are greyed because there is no authentication required for NFS protocol, you just have to add your Rpi's IP address to the allowed ones in your NAS control panel.
Btw, I use a Synology DS214 and it works flawlessly. Please remember that with NFS protocol you have to put the right volume index in the path. For example to mount my "Music" folder in the NAS I have to put "volume1/Musica" in the path.
I have added the IP to the NFS permissions of the Music folder (Read Only privilege, Squash: No Mapping, and Asynchronous enabled) and yep I'm putting the correct path in but still getting 'access denied'. I've checked the NFS options on the NAS and there's a tickbox enabling NFS v4 which is ticked. There's also a field called NFS v4 Domain, which is blank. Finally there's a tickbox for Apply default UNIX Permissions, and this is ticked. Read/Write packet sizes are both set to 8k. There are no other settings.
If I remember, when I played with Volumio before settling with RuneAudio, I was able to enter a username and password for a NFS share. I'm not really clued up on NFS v3 and v4.