This applies to FM broadcasting, and the broadcasting of music as opposed to exclusively voice. Types of FM broadcasting include low power FCC Part 15 compliant devices and LPFM (Low Power FM) broadcasting such as collage campus radio or community radio. The discussion assumes the use of FCC complaint equipment and operating within the boundary of law for your area. This applies to FM Broadcast Stereo transmission on the 3 Meter FM Broadcast Radio band within the legal limits of power, antenna height, emission regulations, FM Broadcast Radio Frequency Deviation, and other related regulations.
The problem: Your low power FM station is very noticeably quieter than the high power FM broadcast stations you share the spectrum with. When you try to increase the input volume into your transmitter, you either start to produce terrible distortion in the music (poor audio quality), clipping (audio cuts out at peeks), or you exceed the legally allotted bandwidth (deviation) which results in a number of issues including those previously mentioned.
Turn up the receiver: Some people say, "so what? just turn up the radio receiver!" However this is a very short-sighted and unsatisfactory solution. The first and most obvious problem is that if you encounter any kind of interference (due to tropospheric ducting or otherwise) or you forget your volume is high and tune your receiver to another station then you get an unpleasant blast from the speaker, bad for your hearing and the speakers on the receiver! It would be much better of your transmission was at or at least near the same level of loudness as other stations.