You can pile on non-forge mods like you would before: if the instructions say put it in the .jar, then put it into the FTBBETAA/instMods folder. Otherwise, put them where they need to go inside the minecraft folder.
As a specific note, Double Door is distributed poorly (it's a .zip with a folder for client and server separately included inside). There's also a question as to whether it interferes with forge classes (I don't have the time right now to spot check, so just relaying what I've read). You can pull together the client files in the .zip into their own .zip and drop it into instMods, but I make no warranty for how well it'll work or if it'll work.
AudioMod is largely redundant with Forge installed. Unless there's a specific call for it, I wouldn't include it. MAtmos, by the way, runs just fine under Forge/FML without it.
As always with adding non-forge mods, the more and broader the mods you add, the greater you increase the chance of java classes in Forge, FML, or vanilla Minecraft itself getting overwritten that you'll need for something else in the pack and the whole thing falling down and going boom. Nobody can take responsibility for making that work in that circumstance. If it doesn't work, it just doesn't work; folks installing their own .jar mods (outside Optifine) are somewhat expected to know what they're doing and whether anybody else can help will rely on if anybody wants to take the time out to work it out with you.