Black circle with white markup

[Jonas Müthing]

Hello list,

I'm writing a piece in which I have four speakers with megaphones. To  
indicate if the megaphone is used or not I use a circle with an M in it.

\markup { \circle { \pad-markup #.5 \bold M } }

This is supposed to indicate that the megaphone is to be used. If it's  
not to be used I want a white M inside a black circle. Is there any  
way to achieve this with the circle function? I tried

\markup { \override #'(filled . #t) \circle { \pad-markup #.5 \bold  
\with-color #(x11-color 'white) M } }

but "filled" seems to be supported only by \draw-circle

Any ideas?

Best regards,

Jonas