diff --git a/src/sound/README.md b/src/sound/README.md
index 8143aea1ab..673e830d75 100644
--- a/src/sound/README.md
+++ b/src/sound/README.md
@@ -48,4 +48,56 @@ This folder contains the related files for all sound APIs.
 
 ## Documentation of sound design
 
-**Fixme:** The sound design is not yet documented.
+This describes how the code behaves today. It covers the device lifecycle and the threading rules
+around the audio callback; the areas still missing are listed at the end.
+
+Each platform provides one `CSound` class deriving from `CSoundBase`, and exactly one of the
+subdirectories is compiled in: `asio/` (Windows), `coreaudio-mac/`, `coreaudio-ios/`, `oboe/`
+(Android) and `jack/` (where JACK is enabled).
+
+### Buffer size negotiation
+
+`Init ( iNewPrefMonoBufferSize )` returns the mono buffer size the device actually accepted,
+which may differ from the one requested. `CClient::Init()` uses that return value to find out
+which sizes a device supports, so it calls `Init()` four times per invocation: once for each of
+`FRAME_SIZE_FACTOR_PREFERRED`, `FRAME_SIZE_FACTOR_DEFAULT` and `FRAME_SIZE_FACTOR_SAFE` to fill
+`bFraSiFactPrefSupported`, `bFraSiFactDefSupported` and `bFraSiFactSafeSupported`, then once with
+the size selected in the settings. Those three flags drive the enabled state of the buffer delay
+radio buttons, and the settings dialog polls them once a second; no signal runs from the sound
+device to that dialog.
+
+A driver may also change the buffer size on its own. `kAsioBufferSizeChange` in the ASIO backend
+and JACK's buffer size callback both report that by calling
+`EmitReinitRequestSignal ( RS_ONLY_RESTART_AND_INIT )`, which reaches
+`CClient::OnSndCrdReinitRequest` and repeats the negotiation above.
+
+### Start, stop and the audio callback
+
+`Init()` is only ever entered with the device stopped. Callers that may be running stop it first
+and restart it afterwards, which is the `bWasRunning` pattern throughout `client.cpp`.
+
+The audio callback runs on a thread owned by the driver. `CSoundBase` inherits `QThread`, but
+nothing here overrides `run()` or calls `start()`, so no such thread exists. Backends keep the
+callback away from a device that is being re-initialised in two ways, and the ASIO backend is the
+exception to both:
+
+| backend | audio callback | ignores the callback while stopped | takes `MutexAudioProcessCallback` |
+|---|---|---|---|
+| JACK | `process()` | yes, `IsRunning()` | yes |
+| CoreAudio (macOS) | `callbackIO()` | yes, `bRun` | yes |
+| CoreAudio (iOS) | `processBufferList()` | no | yes |
+| Oboe | `onAudioReady()` | yes, `!bRun` | yes |
+| ASIO | `bufferSwitch()` | no | no, it uses its own `ASIOMutex` |
+
+`CSoundBase::Stop()` clears `bRun` and then takes `MutexAudioProcessCallback` to wait for a
+callback that is already in flight. The ASIO backend never takes that mutex, so on Windows that
+wait returns immediately and `CSound::Stop()` waits on `ASIOMutex` instead.
+
+### Not yet documented
+
+- device enumeration, and what `SetDev()` does when a device cannot be used
+- input and output channel selection, and the input channel mixing in the callbacks
+- MIDI: device selection, controller mapping and `ParseMIDIMessage()`
+- latency reporting via `GetInOutLatencyMs()`
+- the sound card conversion buffer used when a device's buffer size is not a multiple of the
+  system frame size