If you are connecting your Roku player to your TV and not through another device such as an audio/video receiver , then try the following:
# Check the volume level and mute settings on your television.
# Make sure mute is OFF and that the volume setting is at an appropriately audible level.
# If you are using a composite cable, check that the audio connectors are snugly attached at both ends.
# Connect the Roku player to a different input and see if you can now hear audio. If you can, then there is a problem with the previous input on the TV.
# Try changing the cable.