Hi Jumble,
Welcome to the Apple Community.
Sorry I'm no expert, but your question surely deserves some response.
Re: Is there a step that I'm missing?
Seems things have changed with iOS 15 ... some find an issue with the volume in Apple Maps
(The issue does not seem to have been addressed in recent updates
ie ... About iOS 15 Updates - Apple Support
Apple support have an article online (link below) that may help:
(Seems it is best to adjust the volume while Maps is actually in use.)
Things you can do while following a route in Maps on iPhone – Apple Support
Eg: Says:
"Change other settings for spoken directions
To change the volume, press the volume buttons on the side of iPhone.
In Settings , you can choose whether to pause spoken audio (like podcasts and audio books) during spoken directions and whether to wake iPhone for spoken directions. When iPhone is connected to a compatible car’s audio system, you can also choose to hear spoken directions while listening to the radio.
Go to Settings > Maps > Spoken Directions, then select your options".
Also:
Use Siri, Maps, and the Maps widget to get directions on iPhone – Apple Support
How to change Siri volume - Apple Support
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Discussion thread online about changing the Maps volume:
(NB: The last few repliers, at the end, seem to be satisfied with their solutions. )
iOS 15 Apple Maps navigation voice volume… - Apple Community
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More general Apple Support online articles about audio, ...
Adjust the volume on iPhone – Apple Support
Adjust the mono audio, balance, and phone noise cancellation settings on iPhone – Apple Support
Change iPhone sounds and vibrations – Apple Support
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If needed:
iPhone support Team:
(There's a "Get Support" link at the bottom of their webpage)
iPhone - Official Apple Support
Or
For feedback to Apple:
Product Feedback - Apple
Hope something here will prove useful / helpful !
All the best :-)