, Rohan Drape showed me how to send OSC. The following test, which simplifies the old one and replaces printf with OSC, demonstrates timing as perfect as my ears are able to distinguish.
import Control.Concurrent
import Control.Monad
import System.IO
import Sound.OSC
main = do
hSetBuffering stdout NoBuffering
mapM_ note (cycle [1,1,2])
withMax = withTransport (openUDP "127.0.0.1" 9000)
beat = 60000 -- 60 ms, measured in µs
note :: Int -> IO ()
note n = do
withMax (sendMessage (Message "sin0 frq 100" []))
-- set sine wave 0 to frequency 100
withMax (sendMessage (Message "sin0 amp 1" []))
-- set sine wave 0 to amplitude 1
threadDelay $ beat * n
withMax (sendMessage (Message "sin0 amp 0" []))
-- set sine wave 0 to amplitude 0
threadDelay $ beat * n