These sounds are used frequently by hip hop DJs. They can be police sirens, car horns or the ever popular air raid siren. They can be used rhythmically as if they were part of the song, and they also can be used to help transition between songs. One of the most common uses of the air raid siren occurs at the peak of a strong musical build.
Two of the most common filters used by DJs are low-pass filters and high-pass filters. Low-pass filters cut out the high frequencies of a song, allowing only the low frequencies through. High-pass filters cut out the low frequencies, and allow the high frequencies to pass. Low-pass filters are more commonly used because DJs typically don't want to cut the bass out of a mix. These filters can increase the intensity of musical builds, and you can even create them on the fly with filters.
Flanging is a type of audio delay effect. Basically, it works by playing two copies of the same audio at once, but one is a touch slower than the other. Then, they switch places, so the other audio is slower. This keeps repeating, and causes a rising and falling sound in the track, which is sometimes described as a "whoosh" or "whir" sound.
Like their names imply, risers and swells are sound effects that cause increasing tension. They're typically used to transition to different parts of a song. Risers and swells often transition into the chorus. A drop is another sound effect used for transitions, but it produces the opposite effect of a riser or swell. A drop slows the momentum of a song, and often transitions out of the chorus.