ChannelMergerNode in Web audio API not merging channels

The channelInterpretation means the merger node will mix the stereo oscillator connections to two channels each – but then because you have an explicit channelCountMode, it’s stacking the two-channels-per-connection to get four channels and (because it’s explicit) just dropping the top two channels. Unfortunately the second two channels are the two channels from the second input – so it loses all channels from the second connection.

In general, you shouldn’t need to mess with the channelCount interpretation stuff, and it will do the right thing for stereo.