I think it's not possible, for now...
I hope this function will be implemented.
iOS supports very well sound and container fields with sounds are played but there's no way to put a audio file in it, none to make a new record with built-in microphone... Perhaps Copy-Paste but in iOs I don't know how copy in clicpboard a sound...
From the FMGo TechBrief
Container fields support all image formats supported by iOS: GIF, PNG, JPG, and TIFF. Other file formats including PDF, Word, and Excel can also be stored and viewed. Sound and video formats are supported and will execute when you tap on them. Tapping an empty Container field lets users insert a photo taken from the camera or stored in the device’s image library, or capture a signature. If the Container field has data, users may also choose to Open or Email the contents using the device’s local email client. You can cut, copy, paste, and delete by first tapping
on a container field, which will expose the iOS keyboard, and then tapping again to access the contextual menu containing these menu items. If you have previously copied an image to the iOS clipboard, you are able to paste it into a container field from this menu. You can also paste between containers or from a container field to Mail, for example.