services: alsa-service-type: Fix the loading of 'pulse' plugin.

Fixes <https://bugs.gnu.org/31591>.

* gnu/services/sound.scm (<alsa-configuration>)[alsa-plugins]: New field.
(alsa-config-file): Use 'pcm_type.pulse' and 'ctl_type.pulse' to specify file
paths to the 'pulse' plugin.
* doc/guix.texi (Sound Services): Document this.

Co-authored-by: 宋文武 <iyzsong@member.fsf.org>

doc/guix.texi

@@ -13152,6 +13152,9 @@ See below for details about @code{alsa-configuration}.
 Data type representing the configuration for @code{alsa-service}.
 
 @table @asis
+@item @code{alsa-plugins} (default: @var{alsa-plugins})
+@code{alsa-plugins} package to use.
+
 @item @code{pulseaudio?} (default: @var{#t})
 Whether ALSA applications should transparently be made to use the
 @uref{http://www.pulseaudio.org/, PulseAudio} sound server.

gnu/services/sound.scm

@@ -26,6 +26,7 @@
   #:use-module (guix packages)
   #:use-module (guix records)
   #:use-module (guix store)
+  #:use-module (gnu packages linux)
   #:use-module (gnu packages pulseaudio)
   #:use-module (ice-9 match)
   #:export (alsa-configuration
@@ -44,17 +45,31 @@
 
 (define-record-type* <alsa-configuration>
   alsa-configuration make-alsa-configuration alsa-configuration?
+  (alsa-plugins alsa-configuration-alsa-plugins ;<package>
+                (default alsa-plugins))
   (pulseaudio?   alsa-configuration-pulseaudio? ;boolean
                  (default #t))
   (extra-options alsa-configuration-extra-options ;string
                  (default "")))
 
-(define (alsa-config-file config)
-  "Return the ALSA configuration file corresponding to CONFIG."
-  (plain-file "asound.conf"
-              (string-append "# Generated by 'alsa-service'.\n\n"
-                             (if (alsa-configuration-pulseaudio? config)
-                                 "# Use PulseAudio by default
+(define alsa-config-file
+  ;; Return the ALSA configuration file.
+  (match-lambda
+    (($ <alsa-configuration> alsa-plugins pulseaudio? extra-options)
+     (apply mixed-text-file "asound.conf"
+            `("# Generated by 'alsa-service'.\n\n"
+              ,@(if pulseaudio?
+                    `("# Use PulseAudio by default
+pcm_type.pulse {
+  lib \"" ,#~(string-append #$alsa-plugins:pulseaudio
+                            "/lib/alsa-lib/libasound_module_pcm_pulse.so") "\"
+}
+
+ctl_type.pulse {
+  lib \"" ,#~(string-append #$alsa-plugins:pulseaudio
+                            "/lib/alsa-lib/libasound_module_ctl_pulse.so") "\"
+}
+
 pcm.!default {
   type pulse
   fallback \"sysdefault\"
@@ -67,10 +82,9 @@ pcm.!default {
 ctl.!default {
   type pulse
   fallback \"sysdefault\"
-}
-"
-                                 "")
-                             (alsa-configuration-extra-options config))))
+}\n\n")
+                    '())
+              ,extra-options)))))
 
 (define (alsa-etc-service config)
   (list `("asound.conf" ,(alsa-config-file config))))