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.

You have some way of reaching current users of the app, right? (It is 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.

("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.")