All entities double spawn in multiplayer. Supposedly a gamerule class method needs to be stubbed out because it's running spawn code twice? We don't have the source code to HL:S, so we have no idea.
Sounds from Half-Life 2 clash with Half-Life. Doors during the train ride should not sound like a door handle is being pulled. The footsteps are all from Half-Life 2 as well and clash with the lower sampling rate sounds from Half-Life.