The ornamental design for an audiomixer, as shown and described.
FIG. 1 is a front isometric view of the audio mixer of the present invention;
FIG. 2 is a top view thereof;
FIG. 3 left-side view thereof;
FIG. 4 is a right-side view thereof
FIG. 5 is a rear view thereof; and,
FIG. 6 is a front view thereof.
The broken lines showing environmental structure of the audio mixer form no part of the claimed design. The dash-dot rectangle indicates the extent of the surface being claimed. The dash-dot rectangle does not indicate a claimed feature of the structure. The dash-dot rectangle encloses the claimed subject matter.