Most true silver league protoss won't be able to deal with Mutas. So look on online find a good quick 10-11 muta build and use it. Then once you get your mutas out fly straight into his main and start harrassing while you mass more mutas. Just don't engage anything directly once you see his units come over fly away and attack somewhere else if he has a third run your lings in and attack it with them. Don't start engaging anything till about 40 mutas. Secondly while your harrassing start taking mass bases. This style should work just fine all the way through gold and probally plat.

Some tips that i see alot of low level zerg screw themselves on:

1. Not expanding and droning hard enough in the early game fine a good a build and follow it for the early game if your not following a build your bad and thats why you lose,

2. Don't over pressure a 2 base Protoss locked down and turtling on 2 bases just expand and keep building your army.

3. Don't get stuck on roach/ hydra they are bad in late game especially with the control of a silver league player. you need great micro and the ability to use vipers like a pro and the ability to split your army and harrass all over the place to make this style viable and still then it can be hard.

4. A good late game compositions are ultra/ ling/ infestor, GGlord/ winfestor, Swarmhost/ static D. Again if your stuck on mass roach/ hydra don't qq because you lose to a protos late game army.

5. Mutas when you go mutas don't keep engaging straight up with small numbers use their mobility and keep moving them around until you have large numbers. 

6. Keep doing ling run byes on 3rd 4th and 5th bases.

7. Immortal all ins you hold by scouting and then producing mass untis. once you see immortals coming out with lots of sentries cut drones and start producing mass roach or hydra if you have it and you should be fine.

8. Scout there is no reason EVER you shouldn't have lings patrolling around the map and at the 3rd and 4th of the protoss NO REASON WHAT SO EVER. Furthermore, at silver level there not going to be good at clearing overlords so have them spread out everywhere especially dead space. Good spaces to have them our
       1. 1 around the dead space around main.
       2. another around the dead space around natural to check for tech and units moving out
       3..another over obstacles or dead space around comonly patrolled paths
       4. at far way expansions to check for hidden expos
       5. VERY IMPORTANT dead space on either side of your base cluster to check for drops.