[iOS][realtime voice] Mute can silence assistant audio and leaving the app stops speech

Summary

The realtime voice mute control sometimes silences not only the user's microphone input but also the assistant's voice output. In addition, when leaving Codex on iPhone while a response is being produced or spoken (for example after the thinking phase), the assistant stops speaking instead of continuing or clearly indicating that background audio is unsupported.

Environment

• Host: macOS on Apple Silicon
• Codex/ChatGPT desktop app: 26.727.51351 (build 6119)
• Client: ChatGPT iOS app using Codex remote voice

Steps to reproduce

1. Start a realtime voice session from the iPhone.
2. Tap the mute control during the session and observe both microphone input and assistant output.
3. Start a user turn and leave Codex on iPhone while the assistant is thinking or speaking.
4. Return to Codex and inspect the session state and remaining audio.

Expected behavior

The mute control should mute the user's microphone only, unless the UI explicitly documents a separate output mute. If background continuation is supported, leaving the app should not silently stop assistant speech; otherwise the app should expose a clear audio/session state.

Actual behavior

The mute control can also mute the assistant's voice. Leaving Codex on iPhone can stop the assistant's speech after the turn/thinking phase, even though the realtime session appears to remain active.

Additional context

This looks like an audio-session, output-mute, or iOS background-audio lifecycle issue rather than a thread-routing issue. No private transcripts, cookies, passwords, or user data are attached.