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This issue was discussed on the selinux mailing list, subject line: "acpid cannot run 'amixer' due to SELinux issue?"  (not in archives as of this writing).

With the below local policy, it works as expected - I can, for instance, change the volume as a result of an acpi event (such as pressing a special key on my keyboard).

The AVC denials from the audit log (note: some were hidden by "dontaudit":

------------

type=AVC msg=audit(1450643943.351:1071): avc:  denied  { read } for
pid=17124 comm="amixer" name="controlC0" dev="devtmpfs" ino=13431
scontext=system_u:system_r:apmd_t:s0
tcontext=system_u:object_r:sound_device_t:s0 tclass=chr_file
permissive=0
type=AVC msg=audit(1450645877.188:1497): avc:  denied  { open } for
pid=24344 comm="amixer" path="/dev/snd/controlC0" dev="devtmpfs"
ino=13431 scontext=system_u:system_r:apmd_t:s0
tcontext=system_u:object_r:sound_device_t:s0 tclass=chr_file
permissive=0
type=AVC msg=audit(1450646449.252:1604): avc:  denied  { ioctl } for
pid=26283 comm="amixer" path="/dev/snd/controlC0" dev="devtmpfs"
ino=13431 scontext=system_u:system_r:apmd_t:s0
tcontext=system_u:object_r:sound_device_t:s0 tclass=chr_file
permissive=0
type=AVC msg=audit(1450649676.646:2155): avc:  denied  { getattr } for
 pid=6407 comm="amixer" path="/usr/share/alsa/alsa.conf" dev="sda5"
ino=2231592 scontext=system_u:system_r:apmd_t:s0
tcontext=system_u:object_r:alsa_etc_rw_t:s0 tclass=file permissive=0
type=AVC msg=audit(1450650193.910:2306): avc:  denied  { read } for
pid=8370 comm="amixer" name="alsa.conf" dev="sda5" ino=2231592
scontext=system_u:system_r:apmd_t:s0
tcontext=system_u:object_r:alsa_etc_rw_t:s0 tclass=file permissive=0
type=AVC msg=audit(1450650622.897:2416): avc:  denied  { open } for
pid=9899 comm="amixer" path="/usr/share/alsa/alsa.conf" dev="sda5"
ino=2231592 scontext=system_u:system_r:apmd_t:s0
tcontext=system_u:object_r:alsa_etc_rw_t:s0 tclass=file permissive=0
type=AVC msg=audit(1450651145.995:2552): avc:  denied  { write } for
pid=11799 comm="amixer" name="controlC0" dev="devtmpfs" ino=13431
scontext=system_u:system_r:apmd_t:s0
tcontext=system_u:object_r:sound_device_t:s0 tclass=chr_file
permissive=0

------------

And here is the .te file that audit2allow created:

------------

module allow_acpid_access_sound 1.0;

require {
        type alsa_etc_rw_t;
        type sound_device_t;
        type apmd_t;
        class chr_file { write read ioctl open };
        class file { read getattr open };
}

#============= apmd_t ==============

allow apmd_t alsa_etc_rw_t:file { read getattr open };
allow apmd_t sound_device_t:chr_file write;
allow apmd_t sound_device_t:chr_file { read ioctl open };

------------

Reproducible: Always

Steps to Reproduce:
1. Set up an acpi handler and corresponding script
2. Have that script try to use 'amixer' to alter the volume, etc.
Actual Results:  
The script fails.

Expected Results:  
The script succeeds, just as it does when SELinux is disabled ("setenforce 0")