Vagrant is using VirtualBox under the hood, and presumably the default VB config is to let the VM use two-way audio, so it preemptively opens both the output and mic channel in case the VM needs it.

My solution/fix for this was downgrading VirtualBox back to 5.2.18; seems like something in Virtualbox 5.2.20+ seems to initialize the mic when the VM is booting. Guessing is an unintentional change since there's nothing in the changelogs specifically calling it out.

There's likely some fiddling with the VirtualBox VM settings to prevent it from occurring, but things like Test Kitchen in Chef destroy/create new VMs constantly so any changes get quickly lost.