Add an ON/OFF switch for the multi-track audio mixing feature.

[shogo4405]

Description & motivation

• Add multichannel audio mixer #1386
• I have enabled the ability to toggle the multi-track audio mixing feature on/off. This is a precautionary measure as unexpected issues may be discovered in the initial release. Please enable the feature using FeatureUtil.setEnabled(feature: .multiTrackMixing, isEnabled: true) and provide feedback.

Type of change

• New feature (non-breaking change which adds functionality)

Screenshots:

[github-actions]

Danger has errored

[!] Invalid Dangerfile file: No such file or directory @ rb_sysopen - Sources/IO/IOAudioMixer.swift

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Generated by 🚫 Danger

[shogo4405]

@levs42 FYI.