libobs: mix scene-item audio when item canvas is unset

[summeroff]

Problem

A source's audio plays when the source is set directly as an output channel, but goes silent once the source is wrapped in a scene and the scene is the output source. Reported in streamlabs/obs-studio-node#1493.

Root cause

scene_audio_render_do (libobs/obs-scene.c) mixes each scene item's audio only into a canvas whose pointer exactly matches item->canvas:

if (item->canvas != obs->video.canvases.array[canvas_idx])
    continue;

A scene item's canvas defaults to NULL (scene items are zero-allocated; it is only set via obs_sceneitem_set_canvas, e.g. osn's sceneItem.video). With a NULL canvas the comparison never matches, so the item's audio is dropped on every canvas — while its video still renders, because the video path (scene_video_render) already special-cases NULL:

if (render_canvas != item->canvas && item->canvas != NULL)
    continue;

The asymmetry dates to the video path gaining its != NULL guard ("Fix video mix canvas identity", #720) while the audio path — rewritten separately in da3255b9 — did not.

Fix

Give the audio path the same escape hatch: a NULL item canvas means "mix into every canvas".

Test

Regression test mirroring the exact obsproject#1493 repro (scene-wrapped ffmpeg_source with the item canvas left unset must record non-silent audio): streamlabs/obs-studio-node PR — link added below.

Notes

• One-line behavioral change plus a short comment; matches the file's existing wrapping.
• Not built locally — verified by inspection against the parallel video path; relying on CI for the libobs build.

🤖 Generated with Claude Code

[summeroff]

Regression test for this fix: streamlabs/obs-studio-node#1724