What should I do if I don't hear the voice directions from my Nokia Astound device?

Actual navigation voice commands come from phone loudspeaker. First check the volume is set to loud enough.

Ensure your Voice Language for Navigation is downloaded/installed to the device.  You can check this by selecting:

Menu > Applications > Maps > Settings icon > Navigation > Drive guidance and/or Walk guidance> select "English (US) - male" as an option for Drive guidance and English (US) with street names for walking guidance.