[APU] Fix mission-audio silence: ALSA underrun keepalive + guest_audio_flags

Two Linux mission-audio fixes for Project Sylpheed (audio played in
intro/menu but died when a mission finished loading, never returning):

- alsa_audio_driver: when the guest stalls (mission "Preparing for Sortie"
  load) and the ring buffer empties, feed silence to keep the PCM alive
  instead of sleeping. Previously the small buffer drained, XRUN'd, and
  playback never recovered (matches the known "audio muted permanently").
- xconfig: make the guest speaker config a cvar (guest_audio_flags,
  default Digital Stereo) instead of hardcoded Dolby Digital surround.

(The practical fix on this box also needed log_mask=13 to stop kernel log
spam starving the audio pipeline during missions — config, not code.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/xenia/kernel/xconfig.cc

@@ -9,6 +9,7 @@
 
 #include "xenia/kernel/xconfig.h"
 
+#include "xenia/base/cvar.h"
 #include "xenia/base/logging.h"
 
 #include "xenia/base/filesystem.h"
@@ -19,6 +20,17 @@
 
 #include <ranges>
 
+// Speaker configuration reported to the guest (XCONFIG_USER_AUDIO_FLAGS).
+// Default = 0 (Digital Stereo): the Linux/ALSA APU driver's 5.1 (6-channel)
+// path yields silence in some titles (e.g. Project Sylpheed missions), while
+// stereo output works, so stereo is the safe default. Set to 0x00010001 for
+// Dolby Digital + Pro Logic surround.
+DEFINE_uint32(guest_audio_flags, 0,
+              "Speaker config reported to the guest (XCONFIG_USER_AUDIO_FLAGS): "
+              "0 = Digital Stereo (safe on Linux/ALSA), 0x00010001 = Dolby "
+              "Digital + Pro Logic surround.",
+              "APU");
+
 namespace xe {
 namespace kernel {
 
@@ -94,7 +106,7 @@ void XConfig::SetDefaults() {
   xconfig_data_.user.language = static_cast<uint32_t>(XLanguage::kEnglish);
   xconfig_data_.user.country =
       static_cast<uint8_t>(XOnlineCountry::kUnitedStates);
-  xconfig_data_.user.audio_flags = DolbyDigital | DolbyProLogic;
+  xconfig_data_.user.audio_flags = cvars::guest_audio_flags;
   xconfig_data_.user.av_pack_hdmi_sz = XHDTVResolution.at(1).to_host();
   xconfig_data_.user.av_pack_component_sz = XHDTVResolution.at(1).to_host();
   xconfig_data_.user.av_pack_vga_sz = XVGAResolution.at(3).to_host();