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. …
- 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)
- 192 kbit/s — DAB (Digital Audio Broadcasting) quality.
- 224–320 kbit/s — Near CD quality.