Can I convert audio signals from an iphone to electrical signals to control another device?
One approach would be to use DTMF (Dual-tone multi-frequency signaling) the same as used by a 'tone dialling' telephone. Although sometimes they can take a little tracking down from regular suppliers Holtek manufacture a range of DTMF receivers / decoders that can be used without a processor, for example the following:

Holtek HT9170 DTMF receiver

Application circuit 1 would be similar to what you require and when a DTMF tone is detected you'll get a binary output on D0-D3 as per page 7 of the datasheet. If you need 16 outputs you could use something like a 74154 4-to-16 line decoder or otherwise if you only require four outputs you could use directly.

For initial testing before you've done the software development I see there's a free app called "DTMF Pad" in the app store that may be useful if you decide to go that way.