I would consider adding more sounds on a regular or semi-regular basis and promoting them as appropriate.  For example, you could have a $2 Valentine's pack, which is cheaper than ANY Valentine's card option out there and packs some novelty value.<p>You have some way of reaching current users of the app, right?  (It <i>is</i> for messaging, after all.)  Spend an hour or two and a small amount of money creating a Valentine Pack  (preferably something with "if you receive this, you'll want to show it to your friends" value -- "I love you" not so much, but something quirky/whimsical/hilarious probably), ping all your users with a free sample.<p>("This is a Welfare Valentine, for people who don't have somebody to give them a real Valentine.  Happy Valentine's Day, you love-lorn person, you.  Say, know someone who should be getting a REAL Valentine?  We'll hook you up for $1.99, or alternately, you can send them a Welfare Valentine for free.")