Playing muted strings is a percussive effect with lots of attack, hardly any sustain, and no clearly discernible pitch (in theory). A section of vocalists could achieve a similar effect by using:
plosives (unpitched "T" or "P" sounds)
thumping their chests
That said, sometimes the muted string is really more of a really short ...
Muted notes in standard notation are marked by an x like this picture shows:
and the choir could sing/articulate these x notes by a toneless t- k
The example would be interpreted:
dum t-k dum t-k dum dum dum dum
Your guitar is now a Bb guitar = tuned down a major second. The E strings are tuned in D and also all others are a whole tone lower.
The notation is like you would read it if the instrument would be tuned in C. The instrument is transposing these chords from G down to F.
This is quite the same like the music written for all Bb-instruments like trumpet, ...