sound volume varies between sources

Is the volume reduced on the Bluetooth transmitting device?

Output levels are outside Sonos control so some variation is expected.

It would be nice if Sonos remembered the last used volume level for each device.

There are many ways to stream music, and each way affects the sound and quality of the sound you receive.
First of all, Bluetooth is the worst way to stream, it distorts the quality of the sound since the same amount of data cannot be transferred as via Airplay. However, a nice easy way and can be perfectly fine for everyday use.

Music App, when you stream directly from a Music App such as Spotify, Apple Music, etc., the quality of the music you stream can be adjusted, low-medium-high-lossless, although not everyone has lossless. This setting also affects the quality. There is also the option to turn up and down the volume and set sound profiles, which also affect the sound.

You can also stream directly from the SONOS App, where you can integrate your music App so that it streams through this. I'm not a fan of this solution, but for others it works great.

There are also Apps directly from TV or an Apple TV, for example.
(I experience the best sound from my Apple TV)

In addition, there are also settings that can be made in the SONOS app that affect the speakers and thus how the sound comes out of the speakers.

Can't remember if Arc has Speech Enhancement but if you have this setting then turn it off, just a piece of advice from here.

Otherwise, it's "just" to try your way through the different options and find the best way/experience for you.