Audacity is free, open source software for recording and editing sounds. 

Task 1: Removing noise

To use Audacity’s Noise Removal effect, you start by selecting a few seconds of noise. Then go to Effect -> Noise Removal… and click Get Noise Profile. The dialog closes and you return to your file. Select the whole track and again, open the Noise Removal dialog. This time you can chose the degree and you can perform some fine-tuning.

When you are happy with the result, click Remove Noise and voilà, your noise is gone.

In my example I also applied another effect called Normalize in order to boost the volume a little bit.

Example Before (strong background noise): mixed.mp3 – Before

After noise removal: mixed. mp3 – After

Task 2: Experimenting with Bit Rates

Definition Bit Rate (http://en.wikipedia.org/wiki/Bit_rate):

In digital multimedia, bit rate often refers to the number of bits used per unit of playback time to represent a continuous medium such as audio or video after source coding (data compression). The size of a multimedia file in bytes is the product of the bit rate (in bit/s) and the length of the recording (in seconds), divided by eight. In case of streaming multimedia, this bit rate measure is the goodput that is required to avoid interrupts. … 

Audio (MP3)

• 32 kbit/s — MW (AM) quality
• 96 kbit/s — FM quality
• 128–160 kbit/s — Standard Bitrate quality; difference can sometimes be obvious (e.g. bass quality)^{[citation needed]}
• 192 kbit/s — DAB (Digital Audio Broadcasting) quality.^{[citation needed]}
• 224–320 kbit/s — Near CD quality.

Examples beispiel.mp3 – 16 Bit beispiel.mp3 – 16 Bit beispiel.mp3 – 128 Bit beispiel.mp3 – 128 Bit beispiel.mp3 – 192 Bit beispiel.mp3 – 192 Bit

Task 3: Podcast

For this task, I tried to overlay the songs “Put Your Hands Up For Detroit” and “Let Me Think About It” by Fedde Le Grand in Audacity. Here is what I did: 1. File -> Open … “Put Your Hands Up For Detroit.mp3″ 2. Go to Project -> Import Audio… and select the second song ”Let Me Think About It.mp3″. 3. Adjust Tempo by selecting a track and going to Effect -> Tempo… This changes the tempo without changing the pitch, so your song won’t sound like Mickey Mouse. It may take some trial and error until you get the right tempo. 4. With the aid of the Time Shift Tool you can easily move the tracks until they match. The Envelope Tool can be used to create a nice fade in and fade out effect. Result: Due to legal issues I’m not allowed to publish the result here. If you’ve got further questions, just leave a comment or contact me.