Because we live so far apart me and my players have been using Roll20 for the past year and it's working fine. I really like the fact that I can play music and/or soundeffects while we play but I'm trying to go a bit further. I can't alter my voice much and I'm terrible with accents. Does anyone know any program or software in which you can alter your voice, add filters or something like that? My first bet was some music-editing program but it doesn't seem that there's much I can use from there besides from some echo-effect...

There are a couple ways to accomplish this, although there will be a money requirement. There are a couple ways to "hijack" your audio stream to run it through processors to do what you propose, but I have found that technology unreliable and clunky, even for a guy who's been at this audio thing a while.

Another way is to use a microphone that can be plugged into a device that can perform the changes you like before the audio enters the computer. This, however, is delving into musician's gear and also has a money requirement. Being a musician and already having said gear, this is the route I chose.

If you have a smartphone, check out the apps in the store. I know Google Play has a bunch of free voice altering apps. They might do what you need.

Screaming bee.