Convert Audio File to MP3

We have the following audio file (.RAS file) from a DVR attached. We need it converting into an MP3, the total length should be 4 minutes, 58 seconds. The recording should contain beeps every 2 seconds. The first developer to send a successfully converted file which is correct will be awarded the job. We then require the developer to provided details of how we can do this ourselves (so we can replicate results) in the future using free libraries (ffmpeg etc or similar) which can be used in a simple script. We have had a number of developers who have promised to do this, but failed, therefore once we receive the mp3 version (so we know that you can do the work it will be allocated - you can then provide further details after receiving the work). We are looking to build long term relationships with developers, thanks!