Toggle mute selected tracks in Pro Tools

Try something like this (untested):

let tracks = sf.ui.proTools.selectedTrackHeaders.map(t => ({
    track: t,
    isMuted: t.isMuted,
}));

//Find the index of the currently unmuted track
let unmutedIndex = tracks.findIndex(t => !t.isMuted);

//Now mute that track
if (unmutedIndex >= 0)
    tracks[unmutedIndex].track.trackSetMute({ targetValue: 'Enable' });

//Find the next index of the track to unmute
let nextIndex = unmutedIndex + 1;
if (nextIndex >= tracks.length)
    nextIndex = 0;

//Unmute the next track
tracks[nextIndex].track.trackSetMute({ targetValue: 'Disable' });