The ornamental design for a ceiling-mounted speaker, as shown and described.
FIG. 1 is a bottom front right side perspective view of a ceiling-mounted speaker showing the new design.
FIG. 2 is a top front right side perspective view thereof.
FIG. 3 is a front view thereof.
FIG. 4 is a rear view thereof.
FIG. 5 is a left side view thereof.
FIG. 6 is a right side view thereof.
FIG. 7 is a top view thereof.
FIG. 8 is a bottom view thereof; and,
FIG. 9 is a bottom front right side perspective view thereof.
The broken line showing of exemplary unclaimed environmental structures in FIG. 9 is included for illustrative purposes only and forms no part of the claimed design.