Your TV remote should be able to control the volume of the Arc.  Because you have a HDMI-ARC/eARC connection established between the TV and Arc, the TV is supposed to pass that volume command it receives back to your soundbar. This would not work if the TV is off.

The Arc also has it’s own IR sensor, so if you have a programable IR remote, you should be able to use it it with Sonos after configuration.

https://support.sonos.com/en-us/search?q=remote

I probably did not explain this right.  The eARC works fine with the TV.  When I turn the TV off and play music I am trying to use an old remote I have laying around.  I programmed it to the Arc, but it does not seem to register volume changes when Spotify is playing.  I would like to be able to turn the TV off, play Spoify, then use the old remote for volume only since I can’t just click the side of my iPhone anymore.  

In addition to the obvious audio and video signals, HDMI also passes CEC (Consumer Electronics Control) data. The idea with CEC is that when a user drops a disc into a DVD player, the necessary units power up and select appropriate inputs. When connected to ARC via HDMI, the TV will ignore Volume commands sent by the TV’s remote and pass the Volume commands to ARC via the CEC buss. When the TV is powered down or in standby, TV Volume commands are not passed through via CEC.

If you want to use the TV (or any other remote) remote to send IR Volume commands, set the TV remote to control the Volume on an arbitrary A/V system. Just pick a A/V model that is not nearby. Now the TV remote will send IR commands and ARC can be trained to respond to these commands.

Does the old remote work to control volume under any condition?

According to the article below,  you can only use one remote.  I assume that means that since you have the remote connected via HDMI-ARC, the IR sensor on the Arc is turned off?

https://support.sonos.com/en-us/article/control-sonos-home-theater-with-your-tv-remote

I use my normal Roku TV remote through ARC and that works fine.  The TV sends the ARC signal to the Arc (too many arc’s in here :)).  I have programmed and old remote I have to the Arc.  It registered it and seemed to work for a bit, but it appears that once the Arc gets an ARC signal it shuts down the IR?  

ARC has no idea which remotes(s) send IR commands. You can train multiple IR remotes to control Volume on the same imaginary A/V unit and train ARC to respond to these Volume commands.

Well, one thing you get with the volume control via HDMI-ARC/eARC (really CEC commands) is that the TV always knows and can display what the volume is.  So it does make some sense to disable IR controls when CEC control is active so that the TV accurately knows the volume.  Then again, I believe the Sonos Arc can communicate volume changes back to the TV.

Normally the TV’s internal speakers are disabled while the TV is sending audio via HDMI-ARC (or eARC) and Volume commands are passed on over the CEC buss. In response to Volume commands ARC will return a numeric Volume level to the TV. Some TV’s will display this numeric information, others will ignore it.

An old remote -- or multiple remotes -- putting out an IR code different to the TV’s should work fine. They’ll be able to control TV audio as well as music on the Sonos unit. I’ve used this scheme, with a couple of suitably configured universal remotes, for years and years.