The lack of sound glitch w/the LAPDOCK 500 has recently been discovered & discussed in the MOTOROLA FORUMS @
https://forums.motorola.com
Also,I've discovered that MEDIA VOLUME is turned all the way down AFTER undocking.
Even after turning the volume up in the SETTINGS>SOUND>VOLUME>MEDIA,the volume remains all the way down.The only way to re-enable MEDIA VOLUME is to re-boot & return to SETTINGS>SOUND>VOLUME>MEDIA & adjust as desired.
I've also posted these findings in the MOTOROLA forum listed above,in the MOTOROLA WEBTOP,DOCKS,& LAPDOCKS section.