OK, here goes.
1. Copied the FSX GCA folder, GCA_dsd_fsx_xml_sound.gau, and "Add to FSX panel.cfg.txt" files into the B737_800/panel folder.
2. Edited the panel.cfg file in that folder by adding/editing:
[Window Titles]
window00=Main Panel
window01=Radio Stack
Window02=GPS
Window03=Throttle Panel
Window04=Overhead Panel
Window05=Trim Panel
Window06=PFD
Window07=MFD
Window08=EICAS
Window09=Mini Panel
Window10=GCA
[Window10]
Background_color=0,0,0
size_mm=300,340
pixel_size=300,340
visible=1
ident=10010
position=8
type=special
gauge00=GCA!zzDAT37, 0,0,280,264
gauge01=GCA!apsmall, 40,266,202,55
[VCockpit01]
size_mm=1024,1024
pixel_size=1024,1024
texture=$737_1
background_color=23,17,52
gauge01=B737_800!overhead_popup, 4,4,581,529
gauge02=B737_800!pfd, 6,536,260,260
gauge03=B737_800!trim_popup, 6,801,194,154
gauge04=B737_800!annunciator_speed_brake_armed, 6,961,51,28
gauge05=B737_800!annunciator_below_glideslope, 6,994,52,27
gauge06=B737_800!annunciator_panel_1, 64,961,132,40
gauge07=B737_800!mfd, 347,536,260,260
gauge08=B737_800!autopilot, 205,801,463,90
gauge09=B737_800!annunciator_panel_gear, 205,895,66,71
gauge10=B737_800!annunciator_le_flaps, 204,974,100,28
gauge11=B737_800!annunciator_auto_brake_disarm, 275,895,47,27
gauge12=B737_800!annunciator_anti_skid_inop, 275,925,37,27
gauge13=B737_800!flap_indicator, 308,960,59,59
gauge14=B737_800!compass, 325,895,91,62
gauge15=B737_800!compass, 370,960,50,50
gauge16=B737_800!clock, 424,896,105,105
gauge17=737-400!Transponder, 589,4,190,78
gauge18=737-400!Nav 2, 589,87,190,78
gauge19=737-400!Nav 1, 589,169,190,77
gauge20=737-400!Comm 2, 589,249,190,78
gauge21=737-400!Comm 1, 589,330,190,78
gauge22=737-400!Audio, 589,411,190,40
gauge23=737-400!ADF, 589,454,190,80
gauge24=B737_800!eicas, 704,541,260,260
gauge25=B737_800!adf_vor_backup, 757,908,104,105
gauge26=B737_800!attitude_backup, 533,896,104,105
gauge27=B737_800!alti_and_asi_backup, 641,909,103,103
gauge28=B737_800!efis_control_panel, 672,815,143,76
gauge29=GCA_dsd_fsx_xml_sound!Sound, 1,1,1,1,/GCA/gcacalls.ini
gauge30=GCA!PAR, 0,0,1,1
gauge31=GCA!APC, 0,0,1,1
3. Started FSX, loaded the Boeing Colors B737-800 sitting on the tarmac at KSAC. Lowered the flaps and the flaps sound was heard.
4. Chose View/Instrument Panel/GCA and the ILS and AP instruments appeared.
5. Clicked NRST, chose KSAC, and chose runway 2-20.
6. Clicked the APC word at the lower left of the AP, it lit up for a moment and then went out:
7. No voice was heard...
8. Double clicked the APP_0.wav sound file and heard "zero" said in Windows Media Player.