For the benefit of others reading this in future, it would have been preferred if a shorter sample was attached that was under the 100 MB file attachment limit. However, I did play with this for about an hour in a couple spots just to give an example to you that may help you learn about the Noise Reduction feature in Audacity. I don't think the results here are that great, but you are dealing with some very bad noise, so this feature will be limited in what it can do. I normally use
Sound Forge, but that's not free. Although this feature in Audacity is very similar to the feature I use in
Sound Forge to sample noise and use that sample to remove noise from the signal.
I don't think the adjustments I made to the last part are great. The audio does still suffer quite a bit. Perhaps more playing around with Sensitivity or Frequency Smoothing settings can improve things. I was trying to be fairly aggressive on the noise, because it's so strong. However, if you are willing to accept more noise, perhaps the music can sound better. This type of tool cannot perform miracles. More advanced tools that can isolate the frequencies of the noise might be more useful, then you could try applying filters to reduce those frequencies. However, that may still cause significant harm to the audio. I am not experienced enough with Audacity to know if that's possible.
The Noise Reduction feature is a 2 step tool, which requires you to first select a clip that is all or mostly noise, if possible. Ideally, you want to find a sample that is silent other than the tape noise/hiss/buzz, including free of background crowd noises (coughs, voices, etc).
In this example, there seem to be some good sampling points.
As an example on an easier part, the beginning, I did the following
select from 2m 32.836s to 2m 33.836s
Select Effect -> Noise Removal and Repair -> Noise Reduction
Click Get Noise Profile
select from 0 to 2m 34.156s
Effect -> Noise Removal and Repair -> Noise Reduction
Set Noise reduction db to 10, Sensitivity to 2, Frequency smoothing bands to 2
click OK
When making the reduction, set Noise to Reduce. But you can use Residue when using Preview to hear how much noise and how much signal (i.e. the music you are trying to minimally effect) is being effected. Ideally, you hear all or mostly noise and very little of the signal you want to keep.
Skipping ahead to the end for a more extreme case, I used the same tool and selected from 25m 03.613s to 25m 04.613s and clicked Get Noise Profile, then selected from 24m 57.489s to end and used Noise reduction of 18, Sensitivity of 2, and Frequency smoothing of 2.
I then repeat the process by making the same noise selection as before, clicking Get Noise Profile, then selecting the same 24m 57.489s to end as before and setting Noise Reduction to 10.
The reason I used multiple reductions is I think it may do less harm to the audio than trying to make one larger adjustment. But experiment for yourself and find out.
Of course, the quiet part at the beginning before the music starts sounds kind of messed up and still has a lot of buzz. I think this is unavoidable when you have noise levels higher than the signal levels. Audacity may not do as well with these cases as some other tools such as Sound Forge, but that's not freeware. After you are done using it for noise samples, you could select such parts and use Generate -> Silence if you think it's not worth trying to preserve the quiet bits.
This Noise Reduction process also reduces the signal a little. So you can try to select the same area and do a volume boost if you need to. Of course, doing this can also boost the noise again. So you could do a 3rd noise reduction after this. When I boost audio, I try not to have the maximum level exceed -6 dB. Sometimes I make the highest part a bit quieter with a dB reduction, and then select everything and raise the dB.
Sometimes the noise is pretty consistent over an entire recording, other times it changes. In this example, it obviously changes. Even when it's not so obviously different, I like to break it down into smaller segments to avoid using a noise sample that's too far away from the part I'm trying to correct. I try to find a good break (bit of silence). Sometimes there's a nice long break, other times it's quite short. I try to get a sample that's just noise, even if it means I have to use a shorter sample.
I noticed some clicks in the audio. You may also try the Click Removal tool for further enhancement.
