Edit Blender VSE Source in External App.

EditExternally.py

import bpy
import os
import subprocess

bl_info = {
    "name": "Edit Source Externally",
    "description": "Open VSE audio/video source with an external program.",
    "author": "Joe Button",
    "version": (1, 0),
    "blender": (2, 73, 0),
    "location": "Sequencer > Menu Header > Strip > Edit Externally",
    "warning": "Highly sketchy",
    "wiki_url": "",
    "category": "Sequencer"
}

# Correct the paths. NB. overwrite sourcefile and close external program for source to be updated in VSE

class OpenStripSource(bpy.types.Operator):
    """Open VSE Strip Source"""
    bl_idname = "vse.openstripsource"
    bl_label = "Edit Externally"

    # Change these if you want to use different external programs or if they have different paths on your computer:
    openers = {
    
        # Download: http://www.ocenaudio.com/
        "SOUND": ("C:\\Users\\User\\AppData\Local\\ocenaudio\\ocenaudio.exe", "{filename}"),
        
        # Download: https://sourceforge.net/projects/vdfiltermod/files/latest/download
        "MOVIE": ("C:\\Users\\User\\Downloads\\VirtualDub2_42154\\VirtualDub64.exe", "{filename}"),

        # Download: https://www.getpaint.net/
        "IMAGE": ("C:\\Program Files\\paint.net\\PaintDotNet.exe", "{filename}"),        
    }

    def execute(self, context):
        c = context.scene.sequence_editor.active_strip
        #print(c)        
        try:
            opener = self.openers[c.type]
        except KeyError:
            self.report({'ERROR'}, "No opener for '%s' strip type" % c.type)
            return {"CANCELLED"}
        if c.type=="IMAGE":
            filename = os.path.abspath(
                bpy.path.abspath(c.directory+c.name)
            )     
        elif c.type=="SOUND":
            filename = os.path.abspath(
                bpy.path.abspath(c.sound.filepath)
            )             
        else:    
            filename = os.path.abspath(
                bpy.path.abspath(c.filepath)
            )
        print(filename)
        opener = [s.format(filename=filename) for s in opener]
        output=subprocess.run(opener)
        return {'FINISHED'}

def menu_func(self, context):
    self.layout.operator(OpenStripSource.bl_idname)


def register():
    bpy.utils.register_class(OpenStripSource)
    bpy.types.SEQUENCER_MT_strip.append(menu_func)


def unregister():
    bpy.utils.unregister_class(OpenStripSource)


if __name__ == "__main__":
    register()