[Story] EX: Voice message autoplay

Description

Context: Both EX Android (#6454) and EX iOS (#6089) have implemented autoplay of consecutive voice messages. This issue captures the agreed behavior as a source of truth for testing, since the two PRs were built independently with some diverging discussion.

Acceptance criteria

Core behavior

• When a voice message finishes playing, if the next chronological timeline item is also a voice message, it automatically starts playing.
• The chain continues until either a non-voice-message item is encountered, or the end of the timeline is reached.
• Non-message timeline items that don't represent real content (e.g. day separators, read markers) should be skipped when checking adjacency — they should not break the chain.
• Any other message type (text, image, etc.) between two voice messages does break the chain.

Resuming vs. replaying

• If the next voice message in the chain was previously paused partway through (e.g. left at 0:10 of 0:30), autoplay resumes from that position.
• If the next voice message had previously been played to completion (and is therefore reset to 0:00), autoplay starts it again from the beginning.

Interruptions

• Manually pausing the currently-playing voice message stops the autoplay chain.
• Going back to the room list stops the autoplay chain.
• Manually tapping a different voice message cancels the current chain and starts playing the tapped message instead (which can itself start a new chain).
• Scrolling a playing/autoplaying voice message off-screen does not interrupt playback.

Transition sound

• A short sound plays between two autoplayed voice messages, to signal to the user that a new message has started.
• iOS uses the system "Tink" sound.
• Android currently defaults to the system notification sound as a placeholder — needs a decision on what sound to actually ship, ideally consistent with iOS. Design/product input requested (referenced: @amshakal).

Background behavior

• Autoplay continues even if the app is backgrounded (matching Signal's behavior, not WhatsApp's, which stops in this case). This was discussed and confirmed as intentional, not a bug — see @mxandreas's comparison of WhatsApp vs Signal.
• Android does not currently have general background media playback support; this is being tracked separately in Add media player notification widget and allow background playback #6440.

No user setting (for now)

• There is deliberately no on/off toggle for this feature at launch, on either platform, matching WhatsApp/Signal (neither has one).
• Rationale (from @mxandreas): people who use voice messages generally also want autoplay; a setting can be added later if there's real user demand for disabling it.

Open questions

• What sound to use on Android between voice messages.
• Can Android do autoplay in the background or if not at once, will we accept this as a discrepancy.

Leads

• Tech: @bmarty @Velin92
• Design:

Time sheeting

?

Dependencies

• None

Out of scope

• Auto-scrolling to follow the currently playing message

Subtasks

Android

iOS

Rust

Other

Sign-offs

• Android
• iOS

Start by reading the linked Element X Android and iOS pull requests alongside the acceptance criteria, then check the corresponding platform repositories for the missing subtasks and sign-offs. Done means the agreed autoplay behavior is tested on both platforms, with decisions recorded for the Android transition sound and background behavior.