Ultimate Starcraft 2 blog: Zerg Strategy:Ling Roach Muta: Proper Scouting

Build Order

(15) Spawning Pool
(15) Extractor
(16) Queen
(19) Hatchery
(22) Metabolic Boost
(23) Queen
(30) Roach Warren
(46) Lair
(60) Spire
Note: Build orders should never stay static and are subject to change based on what your opponent is doing. Remember, this is a RTS: use your brain, be adaptable, profit.

General Idea

The Speedling Mutalisk build is very strong, however Terran players have started to adapt and are regularly opening with Hellions in order to deal with early Speedlings and hurt our economy. As a response to that you are going to want to get some Roaches early game, this will also protect you from any sort of an early Marine push. From there you want to transition into Speedling Muta simply because it is such a solid build that allows you to harass and contain a Terran opponent.

Notes

Fast expanding has become almost a requirement for many Zerg players, the added economy benefit is great but one of the primary reasons for fast expanding is that extra larva.
When scouting your opponents base the important thing to be looking for aside from what buildings are down, are what attachments are on what buildings. If you don’t know already here is what you can expect in the following scenarios:
Reactor – Factory: Hellions are on the way
Tech Lab – Factory: Many options: Hellions with infernal pre-igniter, Siege Tanks, Thors (look for Armory for the Thor)
Tech Lab – Starport: Expect Banshees in your base shortly
When engaging Thors with Mutalisks you need to be sure to spread them out. Remember Thors do lots of splash damage, so when your Mutalisks are clumped together they take significantly more damage.