Am I right in presuming you are using the "Record" feature on SMS Timeline?
-And yr. microphone is plugged-into the PC's soundcard.....
+ Is there any other amplification for the Mic?
I'm guessing when Done, you will have a waveform on the Timeline and if you "Solo" that track, you hear the playback, with the SMS Meters showing levels.
What levels are you recording?
What level is the "Hiss" - i.e. during non-speech passages ?
(This level represent the amplification circuits in the Microphone path. ).
An alternative, is to Record VO using a handheld SDHC digital recorder - Ones with built-in mics should be nearly (!) hiss-free . . . . this is the technique I use, although to avoid "Handling noises" I plug-in a tiny tie-clip mic - which cost just over £1. . . . . This has been compared with small mics costing 300x the price, so I'm not convinced it is essential to only use a high-quality mic . . . . . Sure, if you are recording a piano, then "any-ol' mic" won't do justice.
I've never been attracted to use the VO "Record" feature in SMS - I wonder if I'm missing something . . . . What do others think?
If I have to re-read a passage, the I make a "Baa-Beep" sound to mark the previous as "bad" . . . . ( making it easier to delete, rather than go through the whole file in Real-time ). . . . Perhaps others have different techniques . . . .?
Imported Audio music-files should be hiss-free, as they will be recorded significantly above the level of the "noise-floor" and because of this their "Quiet-passages" are still well below normal room noises during Replay.
You could tell us what microphone is being used . . . . and how well it is matched to the Sound-card . . . in particular if it's an Electret, is it receiving enough "Plug-in power" - - - and if it's a professional mic. - is it in need of phantom-power?
Recording level has no impact on hiss level. Hiss level is the same during non-speech passages. Hiss level even occurs when recording an audio event when no mike is connected to audio input on computer.
And there is no hiss at all an imported audio music files.
I've done much recording of narration in the past with no problems.
It reads like there is a problem . . . have you tried the idea of recording the Voice-Audio externally ( e.g. to SD card )? This "should" behave like a music-file . . . without the hiss . . . . as you said Imported Music files are clean.
You say the hiss-level is a constant and even without a mic. connected, so I wonder if that means you have another channel that's providing "hiss" - like it could be a combined L+R audio where One is turned-up . . . giving the appearance that a Mono mic-input is very noisy. ? That no-one else has this issue, suggests it's a Set-Up problem, since one can't imagine modern audio circuits are inherently noisy.
Sorry to say, that's about the limit of my thoughts for now.
Hope it's resolved soonest.