implements background audio with mixer

[simllll]

⚠️ depends on livekit/node-sdks#567

Description

Implements the AudioMixer in BackgroundAudio..enables thinkingSounds.

Changes Made

• implemented AudioMixer in BackgroundAudio

Pre-Review Checklist

• Build passes: All builds (lint, typecheck, tests) pass locally
• AI-generated code reviewed: Removed unnecessary comments and ensured code quality
• Changes explained: All changes are properly documented and justified above
• Scope appropriate: All changes relate to the PR title, or explanations provided for why they're included

Testing

Locally tested 👍

Additional Notes

needs the audiomixer, which can be copied over locally though to test early. AudioMixer only needs the AudioFrame from rtc-node.

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Note to reviewers: Please ensure the pre-review checklist is completed before starting your review.

[changeset-bot]

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Latest commit: 10ee1ca

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