The way forward is going to be via the Google-led Web Audio spec, as well as the complementary MIDI device spec - it's not "there" yet, though, since these specs are still non-finalized. But I'm working on building up the playback capabilities as a side project. I made a "true" Soundfont2 + SMF playback engine for Flash/Haxe, which is open-source(zlib) and somewhat usable in real-world: http://spacecreeps.com/apps/triad-synth/index.html
Now I'm working on the next generation of it, since I was still learning my way around audio with that one and made a mess of the architecture. I'm starting off with a Web Audio-for-Haxe project( early stages, you can watch it here https://github.com/triplefox/hxAudio ) - I'll be building off of that to add the synthesis and MIDI parts, so that you have pretty much everything needed for the playback parts of an audio app; recording isn't on my radar yet - especially since the specs are very early still. It will be possible to compile out the lib to pure JS, so you won't necessarily have to be using Haxe if that's not your workflow.
Now I'm working on the next generation of it, since I was still learning my way around audio with that one and made a mess of the architecture. I'm starting off with a Web Audio-for-Haxe project( early stages, you can watch it here https://github.com/triplefox/hxAudio ) - I'll be building off of that to add the synthesis and MIDI parts, so that you have pretty much everything needed for the playback parts of an audio app; recording isn't on my radar yet - especially since the specs are very early still. It will be possible to compile out the lib to pure JS, so you won't necessarily have to be using Haxe if that's not your workflow.