feat: expose local (microphone) audio level events

[sahilsuman933]

Context

A customer reported a call where the user's audio was never captured, which caused repeated long-pause detections and eventually terminated the call. It was neither a network nor a device-permission issue — the OS-level mic input was effectively silent (wrong device selected / input volume at 0). Daily support recommended using a local audio level observer to detect this and surface a "we can't hear you" prompt.

The Slack thread asked whether local audio level events are exposed by the Vapi web SDK. They were not — the SDK only exposed the assistant's (remote) volume-level. This PR adds the local side.

Changes

In vapi.ts:

• Start Daily's startLocalAudioLevelObserver(100) automatically when a call starts (in both start() and reconnect()), alongside the existing remote observer.
• Emit a new local-volume-level event with the user's microphone audio level (a number between 0 and 1).
• Emit a new local-audio-level-observer-error event when Daily reports a local-audio-level-observer-error (surfaced via Daily's nonfatal-error event, where this error type is actually delivered).
• Add public helper methods:
  • getLocalAudioLevel(): number — read the current local mic level on demand (returns 0 when there is no active call).
  • startLocalAudioLevelObserver(interval?: number): Promise<void> — start/restart the observer (e.g. to change the sampling interval).
  • stopLocalAudioLevelObserver(): void
  • isLocalAudioLevelObserverRunning(): boolean

How this helps diagnose the reported issue

If local-volume-level stays at (or near) 0 while the user is talking, the microphone isn't capturing audio (wrong input device, OS input muted, etc.) — distinct from a network/SFU problem (where volume-level/network events would be the signal) or a hard device error (camera-error/nonfatal-error). Integrators can now show a local "we can't hear you" toast.

Testing

Backward compatible — only additive event names, listener types, and methods.

  

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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