Many audio interfaces with more than two inputs (e.g. an Antelope Discrete 4 Synergy Core) offer several input channel pairs — channel 1/2, channel 3/4, and so on. Through getUserMedia, the browser can only specify how many channels it wants (mono or stereo) — not which physical channels those should be. So in practice it always gets whatever channels the operating system reports as “channel 1/2” for that device.
Symptom
If the actual signal (microphone, instrument) is patched to channel 3/4 on the interface, the level meter in the portal shows nothing at all — even when playing at full volume. This isn’t a connection error or a bug in the portal, but a limitation of the Web Audio API: it’s simply listening to the wrong channel pair.
Telling it apart from the sample-rate issue
A sample-rate mismatch (see “Sample rate: why it has to match everywhere”) can look similar, but usually shows up as crackling/dropouts rather than total silence. Check the sample rate of the interface and operating system there first — if the meter is still completely flat afterward, channel routing is the more likely cause.
Fix: route the signal to channel 1/2
Most multichannel interfaces come with their own routing/mixer software (e.g. Antelope’s Launcher/AFV Mixer, Focusrite Control, TASCAM Settings Panel). Patch the input signal you want so it arrives on USB channel pair 1/2 — not 3/4 or higher. Alternatively, on macOS you can check “Audio MIDI Setup” (Applications → Utilities) to see which channels the device offers at all.
Picking a specific channel directly in the portal (“use channel 3/4 instead of 1/2”) isn’t straightforward — that would need a dedicated channel-splitting UI, which doesn’t exist yet. Routing it via the interface is the simpler path for now.