An exhaustive explanation of profile configuration could be found at http://www.serviio.org/component/content/article/10-uncategorised/16-editing-renderer-profiles. This is just an example how to configure audio transcoding for a device that can basically only play LPCM (aka WAV) and MP3 stuff.

I added the following XML snippet to the end (but before the </Profiles> line) of the serviio-1.0.1/config/profiles.xml file:

        <Profile id="mx7" name="Sony CMT-MX700Ni/CMT-MX750Ni" extendsProfileId="1" alwaysEnableTranscoding="true">
                <!-- Detection>
                        <UPnPSearch>
                                <FriendlyName>.*MX7.*</FriendlyName>
                        </UPnPSearch>
                </Detection -->
                <Transcoding>
                        <Audio targetContainer="mp3" forceInheritance="true" aBitrate="128" aSamplerate="44100">
                                <Matches container="flv"/>
                                <Matches container="asf"/>
                                <Matches container="rtp"/>
                                <Matches container="flac"/>
                                <Matches container="ogg"/>
                        </Audio>
                </Transcoding>
        </Profile>

After this, you have to restart serviio and select the newly added profile for your renderer. This is explained in more detail at http://www.serviio.org/component/content/article/10-uncategorised/5-getting-started.