First question, do you hear this hum when playing other audio files? need to isolate the source as it could be a feedback loop or some weirdness going on with your soundcard, etc.
Second, if the hum is part of the source audio file, in otherwords a noisy recording.. a ground loop.. or just the noise floor itself.. when you bring up levels, it will be accentuated.
There is a chance that DC OFFSET will eliminate it. I know Sound Forge has this feature. Sometimes eliminating the DC OFFSET completely removes the hum. There are a lot of other options, but most require plugins, etc. They are..
-Noise Reduction. Sonic Foundry (err Sony) makes a great Noise Reduction plugin. Costs a lot though.
-EQ. If you can isolate the frequency of the hum (Sound Forge Spectrum analysis is great at this), you can EQ it out – but will lose some adjacent frequencies.
-Noise Gate. you can use the Express Dynamics plugin to set the noise gate so it drops the volume of the sound completely between sounds so the overall presence of the hum is reduced.
all in all, the best solution is either DC OFFSET or Noise Reduction. and you will need Sound Forge for either one… akaik