Trying to use some different codecs with our asterisk installation having it setup like so in our sip.conf

disallow=all allow=g726 allow=ulaw

Our only problem is that the g726 codec does not exist it seems as it is always skipped and uses the ulaw codec and when I comment out the allow=ulaw line asterisk falls over and calls cannot be made in or out.
Does anyone know where the codecs are located so I can check they are indeed there?
We are running

For a standard binary install, the codec binaries are in /usr/lib/asterisk/modules, along with all the other modules.

In the source tree, they are in codecs/

The ones that actually get built depend on menuselect.makeopts, which is managed by menuselect.

I see them listed in /usr/lib/asterisk/modules, but as they are not working I take it they will need to be set using menuselect as you mentioned.
Had a look into the menuselect but it did seem like this is only accessible when installing asterisk, would I be correct in thinking that?

What happens if you explicitly load the module (module load) from the CLI?