time streching is used when you have a sample that is in a different bpm to the rest of your track. You then stretch it to fit. Time stretching can also be used as an effect, especially when you are stretching it like 50-100% of its original time, as it makes the sample soundgritty and raw, sometimes i will time stretch a sample then stretch it back just to make it sound heavy and gritty, it is done thusly....
work out how long in seconds a bar is then time strech to fit, as for the options, soundforge for example has many eg...music 1 throu like 8, solo instruments and drums then you pick which preset is what you are streching eg if you are streching a sample that has more than one layer stretch with a music preset, drums? then use a drum preset then stretch away....
i would also say time stretch the whole sample first then chop it...
as for you mentioning radio, that souds like a filter preset rather than a time stretch one...