"How do I control separate sound sources on MacBook Air?: How can I control different sound sources, like spotify and a game sound on MacBook Air? I want to reduce or increase sound separately."
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Managing Audio Per Device:
Unfortunately, this is not a feature of the macOS. You'd have to do so manually, per app, as long as the app has its own volume settings. Elsewise, it's a matter of using Third-Party software, such as this one: https://rogueamoeba.com/soundsource/. This has the option for a free trial, along with a paid version. So, back up your Mac, and then give it a shot.
IMPORTANT: Create a Time Machine Backup of your Mac, prior to downloading this. That way, you can have something to restore it from, should anything go wrong afterwards. You should always create a Time Machine Backup your Mac, prior to, and after, making any major changes to it, such as update and software installs.
Recommend this to Apple:
Do your Part: Inform Apple of this idea. You'd do so by providing Apple Feedback on this. Apple wont get back to you directly, but the more feedback thy receive from its users, the more likely they will know what their customers prefer, and what options to include in macOS versions to come.
- Go Here: Feedback - macOS - Apple
- Select: "Feature Request" for the "Feedback Type"
- Select: "User Interface" for the "Feedback Area"
- Comment: on why you want this to be an option, and how you think it can help others. Include the URL of this Thread.
- Proceed from there as necessary