SLP recordings have audio muted after editing

unfortunately your video was recoded without audio track, so it's not possible to "unmute" there is no recorded audio.

There should be a RECORD MODE button on the remote.

SLP records 3 times longer so 270 minutes.

the record button is turned on--activate on or off--jus like cool edit

You have to unmute it using this procedure.
start -> All programs -> accessories -> Entertainement -> Sound recorder.

Go to Audio properties from the edit in the menu bar.
In the sound recording tab click on volume.
In that select micro phone and increse the volume.
This might fix your problem.

Yes. SP is "Standard Play" and LP is "Long Play", while EP/SLP are short for Extended Play/Super Long Play respectively.

I assume what you get from a recording attempt is garbled video, or snow? Audio OK?

The first thing is to clean the video head. The best way is to do this manually: take the top cover off, and using a lint free cloth moistened with rubbing alcohol, or an electronic wash product, clean the head. The head is a rotating aluminum cylinder located near the backside of the deck assembly. The actual heads are at the bottom of this cylinder- you can see them- just place a finger in the moistened cloth, and turn the head a few rotations. You should be able to see some deposits on the cloth.

Press the REC MODE button until slp shows up in the bottom right side of the tv.

:: Remote Control Overview
1 POWER Press to power the DVD+VCR on or off.
2 TRK+/- While a tape is playing, press to manually adjust tracking
(i.e., to remove white lines from the screen).
3 0-9 Use to select channels and set menu items.
4 SKIP/F.ADV Press to skip ahead in one-minute increments, up to 5
minutes, during VCR playback. Press to advance one frame at a time in
Pause mode.
5 SHUTTLE <</>> Press the 4 and 6 buttons to scan forward and
backward at variable speeds.
6 MODE/REPEAT Press to set the Play Mode to Random or Programmed
Play. Also press to set the Repeat Mode to Title, Chapter/
Track, or A/B Repeat (DVD only).
7 ZOOM Press to zoom in at 2 or 4 times normal magnification (DVD
only).
8 100+/3D SOUND Press to select cable channels 100 and above.
Also press to activate the 3D Sound feature, which creates the illusion of
surround sound with only two speakers (DVD only).
9 DVD Press to set the remote control to operate the DVD deck.
10 MUTE Press to turn audio on or off.
11 VOL +/- Press to adjust the volume.
12 STOP J Press to stop playback.
13 REW AA In DVD mode, press to skip to the previous track. In VCR
mode, press to rewind the tape. Press and hold to scan backward during
playback.
14 REC F Press once to start recording in the VCR deck. Press repeatedly
to add recording time in 30-minute increments.
15 ARROW KEYS Y/B/A/" Press to navigate in the menu system, see
page 15.
16 SETUP/ENTER Press to display the DVD+VCR menu system and
confirm menu settings.
17 SP/SLP/TITLE Press to set the VCR record speed to SP (Standard
Play) or SLP (Super Long Play). SLP is three times longer than SP. Also
press to display DVD title menus (for discs with multiple titles only)

I would first have it cleaned.