This is to change the behavior of ChannelMergerNode when the input changes dynamically. There have been critical issues with the previous implementation. (See related issues for the detail) To address these issues, a new proposal for the ChannelMergerNode was made. The proposal effectively makes the channel order in the merger node static after the instantiation.
Docs: https://github.com/WebAudio/web-audio-api/issues/304 crbug.com/441451 https://github.com/WebAudio/web-audio-api/issues/459 crbug.com/442925 https://github.com/WebAudio/web-audio-api/issues/304#issuecomment-69641498