Is there any good mod for this game, graphics mod, resolution or widescreen mod ? As far as I search, only Rogue Spear does have mod but none for this game.

riticle, armpatch, character, uniforms, and weapon editors
you might be able to backup and edit: Constants.txt

On an old partial locally cached website: planetrainbow six found mention of grey falcon,

found old save file from vg strategies. NATO v2.01(NATO_v201.zip) used to be one.(newer NATO for rogue spear)
ITF 8.2
Lloyds mods, LWM6

People used to mod or add extra missions training levels. pcgamer level
used to have Wilco voice utility

RSB plugin for photoshop ???
LMods_1-5.zip Llyod Mods 1-26 unknown

You could use Hot Key Resolution Changer. for resolution.
Music help/changing:
Maybe if you mount a music CD virtually on D. The music is there in the C:\Program Files\Red Storm Entertainment\Tom Clancy's Rainbow Six\data\sound\Music on the original CD.

On gog version the files are stored in: C:\Program Files\Tom Clancys Rainbow Six\MUSIC

The CD_TRACK files are named in Constants.txt