Going back to the very first post -- there are way to remove that metallic/glassy effect. Goldwave is the most common cause of that. There is an anti-"Electronic Noise" filter in
this SoundForge filter presets pack that often helps.
Restoring audio is not easy. The most common way to filter it to carve out the unstable/noisy frequencies. There are definitely trade-offs for this. Some of the "counter frequency" methods can work too, although they tend to leave odd digital noise artifacts behind. I'd also remind everybody that
restoring is about "making it better" and not "making it perfect". Even the most expensive and specialized forensic software can have trouble with some types of noise.
For working with audio, I suggest SoundForge, Audacity, Goldwave (in limited usage only), and DiamondCut Forensic. I'm not a big fan of Izotope. Many years ago, DartPro was also pretty decent. There are other options. While I didn't see it mentioned in above posts, I disagree that Pro Tools is good for restoring audio -- it's just a fancy editor.
You also need good speakers.
If the source is analog, run it through mixing boards first, to pre-process it. Some things just sound better fixed through hardware.
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