One of the features I'm tossing into UMotd (Two, Revived, Revised, Remixed, whatever I name it) is a random sound when a message screen pops up.
In testing I've been using surface.PlaySound( "/vo/canals/gunboat_herelook.wav" ). This one in which the refugee man helping you arm the gunboat in HL2 says "Here, take a look at this".. I think when looking at a map.
While tinkering in GCFScape, I'm probably going to use the following randomly.
/vo/canals/gunboat_herelook.wav
/vo/canals/shanty_hey.wav
/vo/canals/shanty_gotword.wav
/common/warning.wav
/doors/door_latch3.wav
/npc/roller/blade_out.wav