Found by parsing the results from this bash script:
for define in $(grep define *.h |  awk'{print $2}');
do if ! grep -q $define *.c
then
        echo "$define not found";
fi
done

Soundicons are used for external audio with SSML <audio> tag and for
replacing punctuation names with sound files in LoadConfig().

Currently there's a bug wih soundicon slots: if both LoadConfig and
<audio> are used, the punctuation reserves all slots and no sound from
<audio> is played.

wavefile_ix is always equal to fmt_params->wav_addr.

wavefile_amp is only used once  so it can be set right
before calling DoSample2().

speed.fast_settings[] was introduced in 7b2eecc724 to be used with
Sonic. Looks like after 7b2eecc724 the code related to everything
except fast_settings[0] was removed.

Might be that speed.fast_settings[0] is also irrelevant and could be
removed.

It is set but never used.

Going through files in src/libespeak-ng/, include-what-you-use removed a
few unnecessary includes and included explanations on why a certain
header should be included. This makes tracking globals and dependencies easier.

Running the codebase through IWYU should be repeated after each major
code restIncludes to standard c library weren't checked to avoid
breaking builds with other platforms.

See https://github.com/include-what-you-use/include-what-you-use

There's two variables with the same name:
1) in struct PHONEME_DATA, used by *phdata, *phdata_next and *phdata_prev
2) an unnecessary one that's only being memset() twice but never used
   for anything

Unsigned char *out_start is a global only used in speech.c.

This makes the codebase more consistent. Including speech.h is not necessary anymore in some files. Includes have been removed.

synthesize.h now contains the definitions STRESS_IS_... that should be used with code related to syllable stress.

Note that isBreak and other defines were renumbered so that stress definitions could have values 0-6.

Possible TODOs:
1. Unify with terms used with phonemes, i.e. keywords like isDiminished in compiledata.c and stress_type in phsource/phonemes
2. Add functionality and documentation about STRESS_IS_PRIORITY and STRESS_IS_EMPHASIZED

See issue #8, https://github.com/espeak-ng/espeak-ng/issues/8

There might still be some ints that could be changed to boolean.

Possible regressions in fifo.c:
int fifo_is_busy() and int fifo_is_command_enabled() used to return an int. Now they return a boolean. This might cause problems on systems where stdbool true/false is something else than 1/0.