What is left ventricle posterior wall fibrosis? thick wall or thin? also, why does it cause a murmur? does cause a valve to leak?

Fibrosis: Fibrosis mean "scar". It usually follow a heart attack although there are rarer causes. Scars are usually thinner than the heart muscle they replace. They do not cause heart murmurs. If you have a heart murmur, it's from something else. The key information you should be asking and didn't list: what is your ejection fraction (a percentage - >55% is normal).

Heart disease: Posterior wall fibrosis can cause papillary muscle dysfunction and mitral insufficiency and the murmur of mitral insufficiency.