We're all talking about different things. If your sole goal is to keep sound from getting out (ie, you're drumming and you don't want the neighbors calling the cops), then you just need material. Some material works better than others, but all material will do SOMETHING to dampen the sound. So here are my suggestions, taken with a grain of salt (some of them I've tried for my specific application; some I've just seen or heard about):
1. Drums. Heavy blankets everywhere that you can. Moving blankets are great if you have them. Hang them from the ceiling all around the drumset. If the set is upstairs and you're worried about it bleeding downstairs, the biggest thing is to decouple it from the floor. The tennis ball riser -
- was popular for a while. Never tried it. I almost always put my drumset in the basement. Stuff something dense in the window well to keep the sound from escaping, and pay special attention to any bare walls that have no insulation, particularly if you're in an unfinished basement. Also, you might consider mutes to help quiet the drums when you're practicing, so you can ONLY play full volume when you're recording.
2. Electric guitars. Iso box is the specific solution for this. You build a box out of wood and dense foam so you can completely seal it up with only a couple of cables going in and out. Ideal is mic'ing a passive cabinet so you don't have to worry about heat, but you can also isolate a combo if you come up with a way to circulate air to keep the heat down. You can fake this with the heavy blankets again, but a sealed box is much more effective.
3. Vocals. Material. Moving blankets are expensive, but honestly, I've seen pro bands insulate their personal studios with moving blankets. If you don't have enough to really isolate your screamer vocalist for a recording session, put him or her in an interior closet with clothes hanging all around. Keeps the sound muffled, plus it makes the vocal track quieter (actually all of these solutions will help with that).
4. Bass. Well, bass is the hardest to muffle because it has so much energy. Isolating a bass cab is helpful, but not always easy. I'll be honest, even before I sold my bass rig in favor of modeling and an FRFR for stage monitoring live, I never recorded with it. I always just ran direct and colored the tone from there. Today's modeling is so good that the upside of actually mic'ing a bass amp is minuscule. If you MUST have loud bass "in the room," consider reducing the lowest frequencies when practicing so they won't be assaulting your household or your neighbors constantly.
Hopefully this helps - see if you can find a source for cheap moving blankets (I kid you not - one band I follow has a "pod" that they lower over the drummer... it looks like an umbrella made out of moving blankets and lowered from the 18' warehouse ceiling to about 3' over his head). Keeping things in the basement with ground all around is also a great trick as long as you take care of open points like basement windows and places where pipes pass through the exterior wall.
I should also point out that a lot of home studios employ modeling whenever they can. I use Helix Native in my DAW rather than mic my amp because, honestly, I can't tell the difference (and in my untreated room, the amp usually sounds worse unless I take a lot of time to dial it in). I use synths for all keyboards, and of course I model bass amps. I'll often use drum samples, even when I still had a drumset (though I am buying a new set of drums and building a drum room in the basement, so I'll have to report back how that goes). Lots of tools out there to help you do your thing at any volume. Good luck!