It all depends on how long you want to use your car.
transmission fluid 漏: If your car is an American car, that is quick common after five or six years, and even worse in winter (metal contracts).
修一下大概要多少钱: Expensive, doesn't worth.
My suggestion - learn how to measure transmission fluid level. Buy a bottle of transmission oil, refill it yourself. the cost is around 20 CAD per bottle, and I think one bottle is good enough for one month in winter, and longer in hot summer.
muffler有两个孔，换一个旧的如何 - Cost of labour is around 120 - 150. Why "旧" ? A generic one plus labour is around 400 (more expensive if at LUBE/Canadian Tire). A generic one can last 3 years, original can last longer. (double)
