The following is a simple demonstration of typical denoising tools. The basis is a recording of "Ora e per sempre addio" by Francesco Tamagno. The hiss reduction tool is essentially a volume-dependent frequency filter: large signals are passed unchanged, smaller ones are attenuated in the highs, reducing perceived noise. The denoising tool employs autocorrelation to attenuate brief signals, but in the process can introduce echoing or metallic sound. The program used for this demonstration is Adobe Audition, the successor to Syntrillium's CoolEdit Pro.
The raw file may be heard here; software was used to remove clicks before the experiment. Note that the artifacts that result from processing are not always evident with casual listening and, indeed, may be considered desirable to some listeners especially when an extended work is being auditioned.
E-mail me at mrichter@mindspring.com