The instruction con sord or mute will usually mean a straight metal mute. However there are many different mutes available for the trombone:
clip 1 -
Bucket mute...
clip 2 -
Cup mute...
clip 3 -
Harmon mute...
clip 4 -
Pixie mute...
clip 5 -
Plunger mute...
clip 6 -
Solotone mute...
clip 7 -
Straight mute (metal)...
clip 8 -
Straight mute (wooden)...
clip 9 -
Whispa or practice mute.