iPhone 4 speaker not as loud as another iPhone 4

So the other day i got an iphone 4(philip screws on bottom) from my sister because she didnt want to use it anymore. The problem was that the speaker was very low so i bought a new dock connector and replaced it and now its way louder but not as loud as my sister in law's iphone 4(pentalobe screws at the bottom). Any suggestions on why its like that? Is it because i have the iphone 4 with the philips screws and not the pentalobe one? Please help! Thanks!