Due to regression, after the stereo-to-mono split, both new tracks were assigned the same ID, which should be unique to all tracks. This was causing weird behavior, including data loss and crashes.

Resolves: #7048, #7050, #6886

• I signed CLA
• The title of the pull request describes an issue it addresses
• If changes are extensive, then there is a sequence of easily reviewable commits
• Each commit's message describes its purpose and effects
• There are no behavior changes unnecessary for the stated purpose of the PR

Recommended:

• Each commit compiles and runs on my machine without known undesirable changes in behavior