There is a “quick” and a “full” Trueplay. Newer speakers with inbuilt microphones can do the former but you need an appropriate IOS device for the latter. Some folk will borrow an iPhone to achieve this. 

My Arc looks like it has a microphone.  What is the quick way ?

ps I think you have confirmed; i need an iphone.  I wonder why not Android 

To quote:

Trueplay tuning is not supported by:

• Android devices
• Devices running a beta version of iOS or iPadOS

Android devices and devices running beta versions of iOS or iPadOS can use the quick tuning function of Trueplay, if available. Quick tuning is currently supported by Arc Ultra, Era 100, and Era 300.

I have an Arc

Because iPhones' micorphones are standardised. There just too many different Android phones out there.

https://support.sonos.com/en-us/article/automatic-trueplay-tm

https://support.sonos.com/en-us/article/tune-your-sonos-speakers-with-trueplay

I have an Arc

And, as you say, you have an Arc. It’s got a microphone for voice control, but it’s too old a device to support automatic Trueplay. 

The Arc Ultra has Trueplay (of sorts) using iOS and Android. Unless I have missed something, does this not make a mockery out of the”reason” the standard Arc lacks similar?   At the price point Sonos occupies, a supplied microphone for room tuning would not go astray….

Ah, the Sonos voyage of discovery.    At least it is not Bose; ‘no highs, no lows, must be Bose’

If you’ve looked at details of the “full” Trueplay, you’ll know it involves moving around your room, waving the IOS device, whilst the speakers emit a series of tones. This is used to create an audio “map” of the room’s acoustics and the speakers’ out put is adjusted accordingly. IOS devices apparently use a “standard” microphone in each model of device, which enables Sonos to do the math to create the sound-mapping. 
 

With newer devices such as the Arc Ultra a simplified version of Trueplay has been introduced. Since the speakers can’t move, the room audio map cannot be as detailed as with full Trueplay. But, it does potentially offer improved sound reproduction. To do so, Sonos have presumably standardised the microphone on the supported speakers. 
 

To introduce and backdate that to include earlier speakers with microphones presumes Sonos standardised microphones in those models. In fact they may well have had multiple suppliers to manufacture to maximise profit or to maintain availability of parts in the manufacturing process rather than standardise on a single supplier. 
 

You ask, in effect, “why not supply a microphone for room tuning?” What should it plug into? What interface(s) or connectors will it have? There are so many Android phones with different interfaces. Supplying a microphone is not really viable. 
 

For many of those without an iPhone, most have a friend with one, and it only takes a few minutes to borrow it, install the Sonos app and go through the Trueplay process.