When the map starts and when the MOTD shows up are two very different things. The MOTD shows up every time a player joins your server. When the map starts, there is technically nobody in the server (the first players will join shortly after it starts).
If you want to base it on the MOTD, you can use client_putinserver() (might want to consider using a set_task to ensure that they will hear it). This will play the sound for each player every time they join the server.