Anyone know where I can get a unit to make multiple lights flash with the musical beat with a 12v system?

I am not sure if you wanted interior or exterior lights but a great place to shop is http://www.slickcar.com they have everything that flashes for any vehichle you want and the lowest prices you will find anywhere. They have very helpfull customer service that will advice you on just what your custom job needs so that you don't end up a wire harness short or the wrong switch.

RECON, the premiere manufacturer of aftermarket lighting and accessories now offers a new 90-watt high-intensity strobe, the brightest plug-n-play strobe lights commercially available. Designed to include 2 extra ports so adding 2 more super high-intensity 90-watt strobe lights to your kit is as easy to install 1-2-3! This new Recon kit boasts a one of a kind Ethernet cable plug-n-play installation with a customizable strobe pattern switch. The switch can be surface mounted anywhere inside your vehicles cab.

You should also check out http://www.streetglow.com for some really cutting edge products.
GOLD SERIES NEON UNDERCAR Kits are the most popular choice for competition-level show vehicle and for extreme neon enthusiasts around the world.
Encapsulated in high impact, water resistant acrylic tubes.
Powerful 9500 Volt transformer with aluminum heat sink for greater heat dissipation.
Pre-wired for sound activation watch the neons 'dance' to the beat of the music.
Limited LIFETIME warranty - including internal glass breakage!!!
Includes:
• Two 30" front and rear tubes 
• Two 51" side tubes 
• 9500 Volt transformer 
• Toggle switch
• All required mounting hardware
• Installation instructions

MUSIC INTERFACE MODULE
Connect up to 12 products allowing them to "dance" to the beat of the music  
Compatible with any 12 volt product (neon or LED) including Gold Series and OPTX Undercar Kits