Wish my dad hadn't given my computer a virus, as I had plenty of reps where my mid-game composition was muta/ling with baneling drops in the mineral line.
The thing is, is that if you decide to go this composition, you need to develop a way to get there. You can't just tech straight to muta/ling; not only is it bad RTS play, but its a not good strategy. A good strategy is safe, secure, and put you AT LEAST in an even playing field.
What I did when I was doing this type of playstyle is that I opened with lings with quick +1 ups. Normal 14gas/14pool opening(you could go 14pool/14gas, don't matter), and pump drones. If you see your opponent expanding, you can take your third, take all 4 of your gases(your going to be REALLY high on minerals), get a third queen to spread creep, and so forth. When your lair is done, your going to make like 2 rounds of zerglings and keep them at the front of your opponents natural, poking and prodding, buying time for your mutas.
Also your drop your baneling nest when you take your 3 gas, just in case of some sort of 4 gate or robo push play.
Once your mutas come out, you harass just like you would ZvT, while taking expansions, getting your 5th and 6th gas up, upgrading ovie speed/ovie drops, harassing with baneling drops on the mineral line, and making TONS of banelings to snipe nexuses. Once you get +2 melee attack, I believe it takes 10-12 banelings to snipe a nexus. You literally have a ball of banelings rolling around the map sniping nexuses.
Very fun style to play but its really risky to 4-6gate early game timings of Toss. But if you have your baneling nest up by the time the 4gate reaches your base, you should be pretty safe.