Hello.
I know I have no say in whatever's been going on now because I haven't been part of the community for a while but I thought I'd make a few suggestions that could be implemented before the game is released on Steam.
I've pushed for Steam integration in the past however adding a game to Steam only boosts the playerbase temporarily.
Proper infrastructure to introduce people to the game to maximize adoption of new players and then to keep players interested for as long as possible. If you don't, you will end up like Quake Live: http://steamcharts.com/app/282440#All where the long term benefit of adding the game to Steam does not exist. Teeworld's fares better than Quake gameplay and graphics wise by being easier and more appealing to get into, but has a huge disadvantage of not being advertised at all outside of Steam.
How to keep as many short-term players as possible?
Here we want to maximize the number of people that play for a few days and don't just log in and log out. Here are a few suggestions in the order of time allocation needed:
- (misc) Disable that menu music on default 
(is it still in?)
- (misc) Turn dynacam off. It makes some people sick...
- Have an easy way to access tutorials. Tutorials include how to play, relevant game types, weapons, relevant mods and how they work, map editor, server hosting, downloading and installing custom skins, etc.
- Contact server hosters and to make a few "official" steam launch servers (EU, NA, BR, etc). Auto favourite them if possible.
- Find a way to keep pub stomping to a minimum (Quake tried something like that by giving a player a full weapon loadup on respawn)
- Have a one button server hoster that will load a default server with votes that will allow you to change gametype and settings. (instead of opening a prompt for an unspecified file, which requires knowledge of what it wants from you, what the server settings are and/or that you can load the default settings by loading a random file)
- Official Teeworlds youtube channel with information + more tutorials. How to move properly, capabilities of grapple launcher, basic grenade techs.
How to keep as many long term players as possible?
People usually play a game for an extended period of time (more than a year) for the social aspect and for competition (see Super Smash Bros. Melee and League of Legends). If people are as excited as I am about the Steam addition, they may want to work harder to help Teeworlds keep as many players as possible. I'm not sure exactly how to foster a community but here are some suggestions anyways... I think steam has a place to advertise events so...
- Advertise voice chat groups and communities (skype? team speak?)
- Somehow encourage creation of clans regardless of level of skill. [GreenNoob] anyone?
- Regularly scheduled / advertised capgames, clan wars, and tournaments.: Just as a note, I wish this was more of a thing. It really is less commitment to say, "I have a game at 5pm I should play" over logging in and hoping enough players will join for a game.
- Have community leaders / established clans to create mid-low level clans for recruiting new players.
- Seeing how the playerbase is so small, having a community leader to step up to be the TO (Tournament organizer) would be amazing for organizing tournaments and leagues for clans. A game like smash bros survived because people stepped up to create something concrete the community could latch on to. (Tournament weeklies/ monthlies)
How to create long term players that would not otherwise exist?
Many players will not stay long enough to become a long term competitive player because they get stomped in a public serve, feel bad, and do not have the competitive drive to start from the bottom to work their way up. You need to facilitate enjoyment for the people with a more casual mindset, and get them to play long enough to get to an intermediate level. My suggestions would be to use a level up system and achievements (see CoD), but I believe no-one would be up for the task for this one.. The prestige system is silly to a competitive player, but works wonders for extending the longevity of the game for casual players.
Honestly, I am cynical about the game's survival no matter what we do but imagining what could be makes me extremely excited. Regardless, I'll be around the help out new players and hope enough stay so we can enjoy this game for much longer. Good luck!
EDIT: I have just looked at past posts and a lot of these ideas have been mentioned already. I have a one more idea so I'm not being redundant. 
(they do require a lot more time investment however)
For keeping short term and mid term players, you might want an offline mode as Nilaya said previously so less experienced players can grind something out and improve on their own. You could have a pseudo offline single-player mode where you use the race gametype to create maps with a clear objective and with a linear increase in difficulty. Examples could be from being a simple obstacle course, to capping the flag/ finishing a race below a certain time. The ideal new next mod for this idea would be a Break the Targets sort of gametype: https://www.youtube.com/watch?v=TwQ94WeXm6g which could introduce a huge amount of depth to the potential single player gametype with the movement and weapon options used in teeworlds.
You are right about the sound, though.