Better late then never to reply is what I thought.
1. If you want to use env_soundscape with a custom sound, you'll have to make yourself a soundscape-file. If you have done that you can use it on your map by naming it soundscapes_MAPNAME.txt and placing it in your hl/scripts-folder. You could also use a ambient_generic which is in my opinion easier to handle. You don't have to make a mini-mod for this.
2. You can change soundeffects by placing sound-files which have the same name and location as the normal sounds in your sounds-folder. Best way to know what name and location a sound has is by seeking it in the GCF-file of your map by using GCFScape which you can find at nem's tools on the mighty internet.