From the Falcon Studios description of the gay porn flick The Guys Next Door:
The grunts and groans [of sex] louden, calling Marcus [Mojo] and Johnny [Torque] back to the scene [an orgy in progress].
That’s inchoative louden ‘become loud, become louder’. It struck me as unidiomatic; despite the parallel with soften, I would have written got loud(er). But it turns out that virtually every dictionary I looked at has it. The OED has the intransitive — inchoative — use from the mid-19th century on, but only one cite (from 1898) for the transitive — causative — use, which it marks as rare.
The inchoative or causative suffix –en is in fact extremely restricted in English — not productive, and limited to only a few sorts of base words: monosyllables ending in obstruents, from the Anglo-Saxon (rather than Latinate) stratum of the vocabulary. Even then, not all eligible bases allow derivatives in this –en; hotten is not attested (heat (up) serves this purpose), and colden is attested but marked by the OED as rare (cool and chill serve this purpose, or better, get cold(er)), and modern speakers reject it. For me, louden is like colden.
