ideal center to go between two sonos fives

Not knowing how you’ve connected, I’ll guess you’re using the headphones outlet, which is a variable level signal and may not be turned up loud enough. With the audio delay you may also have lip sync issues? Either way, the Fives aren’t intended as tv speakers. But they will only play (in stereo) whatever audio mix the tv sends out, according to how the audio is mixed in the film itself. 

An Arc or Arc Ultra will give best quality, but either soundbar provides all front channels, not just centre. Either keep the Fives for stereo music play, or use them as surrounds behind your viewing position. 
 

Sonos don’t (yet?) offer separate speakers for home theatre systems, though many have requested it. 

I’m connected wirelessly to my Apple 4k box

Currently, Sonos can’t receive an AirPlay 2 signal that is greater than stereo. While Apple has just released a new SDK to the public which allows greater, Sonos has not had the opportunity to integrate it. 

Headphone connection is used as line in from the amp for my turntable……. It’s just bugging me a little as to why the vocals are that much quieter than the other sounds coming from the movie

It’s probably how the audio is mixed in the movie, rather than how Sonos (or any other speakers) might play the audio. 

As said above, not the ideal setup but you could Group an Era-100 with the Fives and it would add volume,  but not actually a center channel, just centrally located.

A Play 3 might look better but they are pretty old.

There is a “speech enhancement” feature too on the Arc Ultra, designed to boost vocals. 

As has been stated Fives are not intended to be used as Left/Right speakers for TV audio. Just because you can; given the versatility of Sonos speakers, doesn’t mean you should. The only speakers intended for TV audio in the front position are Sonos sound bars past or present. The only other option is to use a Sonos Amp with wired Sonos branded in-wall speakers or 3rd party speakers in-wall or outboard. 

I don’t like your tone, I suggest you wind your neck in 

Ouch 😂