Modified https://gitlab.freedesktop.org/pulseaudio/webrtc-audio-processing/-/merge_requests/17/diffs?commit_id=c18b3945a1cc for FreeBSD + pffft patch to include altivec.h.
--- webrtc/common_audio/wav_file.cc.orig	2020-11-27 19:30:53 UTC
+++ webrtc/common_audio/wav_file.cc
@@ -14,7 +14,7 @@
 
 #include <algorithm>
 #include <array>
-#include <byteswap.h>
+#include <sys/endian.h>
 #include <cstdio>
 #include <type_traits>
 #include <utility>
@@ -123,7 +123,7 @@ size_t WavReader::ReadSamples(const size_t num_samples
   }
 #ifndef WEBRTC_ARCH_LITTLE_ENDIAN
   for (size_t i = 0; i < num_samples; i++) {
-    samples[i] = bswap_16(samples[i]);
+    samples[i] = bswap16(samples[i]);
   }
 #endif
 
@@ -171,7 +171,7 @@ size_t WavReader::ReadSamples(const size_t num_samples
 #ifndef WEBRTC_ARCH_LITTLE_ENDIAN
   // TODO: is this the right place for this?
   for (size_t i = 0; i < num_samples; i++) {
-    samples[i] = bswap_16(samples[i]);
+    samples[i] = bswap16(samples[i]);
   }
 #endif
 
@@ -229,7 +229,7 @@ void WavWriter::WriteSamples(const int16_t* samples, s
 #else
       std::array<int16_t, kMaxChunksize> converted_samples;
       for (size_t j = 0; j < num_samples_to_write; ++j) {
-        converted_samples[j] = bswap_16(samples[i + j]);
+        converted_samples[j] = bswap16(samples[i + j]);
       }
       RTC_CHECK(
           file_.Write(converted_samples.data(),
@@ -241,7 +241,7 @@ void WavWriter::WriteSamples(const int16_t* samples, s
       for (size_t j = 0; j < num_samples_to_write; ++j) {
         int16_t sample = samples[i + j];
 #ifndef WEBRTC_ARCH_LITTLE_ENDIAN
-        sample = bswap_16(sample);
+        sample = bswap16(sample);
 #endif
         converted_samples[j] = S16ToFloat(sample);
       }
@@ -267,7 +267,7 @@ void WavWriter::WriteSamples(const float* samples, siz
       for (size_t j = 0; j < num_samples_to_write; ++j) {
         int16_t sample = FloatS16ToS16(samples[i + j]);
 #ifndef WEBRTC_ARCH_LITTLE_ENDIAN
-        sample = bswap_16(sample);
+        sample = bswap16(sample);
 #endif
         converted_samples[j] = sample;
       }
--- webrtc/third_party/pffft/src/pffft.c.orig	2020-11-27 19:30:53 UTC
+++ webrtc/third_party/pffft/src/pffft.c
@@ -100,6 +100,7 @@
    Altivec support macros 
 */
 #if !defined(PFFFT_SIMD_DISABLE) && (defined(__ppc__) || defined(__ppc64__))
+#include <altivec.h>
 typedef vector float v4sf;
 #  define SIMD_SZ 4
 #  define VZERO() ((vector float) vec_splat_u8(0))