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.

Edited don't know legal status of mods for game? The gog released files have a no mod file: license.txt that wasn't in the retail release, AFAIK. There were cheat codes and debug key codes, (a tilde menu) in original cd pc version.
F1 - Third Person view.
Press + or - to increase/decrease view window in game. Crouch if needed.
sherman.kmp can be renamed from a blank text file to reset keyboard controls.
There are night vision and other settings for the game controls in the game menu.

A pcgame wiki has link to fov fixes. dgvoodoo wrapper may help add 4:3 support. Can also view sticky in forum thread for Windows 10.

DEBUG MODE
press [Quotation mark] during the game and type "debugkeys" at the chat window.
To deactivate debug keys, press [F10]

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

What is Ubisofts policy on mods? The license.txt had a no-edit policy, MSN Gaming Zone hosted Rainbow Six server rooms with the NATO mod back when they did. It changed the GoCode audio files too... PC gamer made a level that other mods are dependent upon. The readme for 1.04 discussed 3rd party utilities or something. Maybe GOG can get permission to change that license when they fix some bugs/corrupted install of files?

Found some of GOG POLICIES: https://support.gog.com/hc/en-us/articles/16033687971613-Infringing-Content-and-Copyright-Policy-effective-from-17-February-2024?product=gog

i think some other games allow mods. Don't even know if add-on levels are welcomed by the new developer for this game?