Apparently 60ms is too much for some cards.

Index: espeak-ng/src/libespeak-ng/speech.c
===================================================================
--- espeak-ng.orig/src/libespeak-ng/speech.c
+++ espeak-ng/src/libespeak-ng/speech.c
@@ -248,7 +248,7 @@ ESPEAK_NG_API espeak_ng_STATUS espeak_ng
 static espeak_ng_OUTPUT_MODE my_mode = ENOUTPUT_MODE_SYNCHRONOUS;
 static int out_samplerate = 0;
 static int voice_samplerate = 22050;
-static int min_buffer_length = 60; // minimum buffer length in ms
+static int min_buffer_length = 49; // minimum buffer length in ms
 static espeak_ng_STATUS err = ENS_OK;
 
 t_espeak_callback *synth_callback = NULL;
