a simple alsa volume control program, see http://stackoverflow.com/questions/6787318/set-alsa-master-volume-from-c-code

Makefile

# build helloworld executable when user executes "make"

volumecontrol: volumecontrol.o
	$(CC) $(LDFLAGS) -lasound volumecontrol.o -o volumecontrol
volumecontrol.o: volumecontrol.c
	$(CC) $(CFLAGS) -c volumecontrol.c

# remove object files and executable when user executes "make clean"
clean:
	rm *.o volumecontrol
	

openwrt-Makefile

##############################################
# OpenWrt Makefile for helloworld program
#
#
# Most of the variables used here are defined in
# the include directives below. We just need to 
# specify a basic description of the package, 
# where to build our program, where to find 
# the source files, and where to install the 
# compiled program on the router. 
# 
# Be very careful of spacing in this file.
# Indents should be tabs, not spaces, and 
# there should be no trailing whitespace in
# lines that are not commented.
# 
##############################################

include $(TOPDIR)/rules.mk

# Name and release number of this package
PKG_NAME:=volumecontrol
PKG_RELEASE:=0.1


# This specifies the directory where we're going to build the program.  
# The root build directory, $(BUILD_DIR), is by default the build_mipsel 
# directory in your OpenWrt SDK directory
PKG_BUILD_DIR := $(BUILD_DIR)/$(PKG_NAME)


include $(INCLUDE_DIR)/package.mk



# Specify package information for this program. 
# The variables defined here should be self explanatory.
define Package/volumecontrol
	SECTION:=utils
	CATEGORY:=Utilities
	TITLE:=volumecontrol -- a simple volume control program
	DEPENDS:=+alsa-lib
#	DESCRIPTION:=\
#	If you can't figure out what this program does, \\\
#	you're probably brain-dead and need immediate \\\
#	medical attention.
endef


# Specify what needs to be done to prepare for building the package.
# In our case, we need to copy the source files to the build directory.
# This is NOT the default.  The default uses the PKG_SOURCE_URL and the
# PKG_SOURCE which is not defined here to download the source from the web.
# In order to just build a simple program that we have just written, it is
# much easier to do it this way.
define Build/Prepare
	mkdir -p $(PKG_BUILD_DIR)
	$(CP) ./src/* $(PKG_BUILD_DIR)/
endef


# We do not need to define Build/Configure or Build/Compile directives
# The defaults are appropriate for compiling a simple program such as this one


# Specify where and how to install the program. Since we only have one file, 
# the helloworld executable, install it by copying it to the /bin directory on
# the router. The $(1) variable represents the root directory on the router running 
# OpenWrt. The $(INSTALL_DIR) variable contains a command to prepare the install 
# directory if it does not already exist.  Likewise $(INSTALL_BIN) contains the 
# command to copy the binary file from its current location (in our case the build
# directory) to the install directory.
define Package/volumecontrol/install
	$(INSTALL_DIR) $(1)/bin
	$(INSTALL_BIN) $(PKG_BUILD_DIR)/volumecontrol $(1)/bin/
endef


# This line executes the necessary commands to compile our program.
# The above define directives specify all the information needed, but this
# line calls BuildPackage which in turn actually uses this information to
# build a package.
$(eval $(call BuildPackage,volumecontrol))

volumecontrol.c

#include <alsa/asoundlib.h>
#include <alsa/mixer.h>

int main(int argc, char *argv[])
{
    long volume;
    if (argc != 3) {
        printf("Usage: %s snd_card_name vol\n",argv[0]);
        return 1;
    }
    volume = atol(argv[2]);

    long min, max;
    snd_mixer_t *handle;
    snd_mixer_selem_id_t *sid;
    const char *card = "default";
    //const char *selem_name = "Master";

    snd_mixer_open(&handle, 0);
    snd_mixer_attach(handle, card);
    snd_mixer_selem_register(handle, NULL, NULL);
    snd_mixer_load(handle);

    snd_mixer_selem_id_alloca(&sid);
    snd_mixer_selem_id_set_index(sid, 0);
    snd_mixer_selem_id_set_name(sid, argv[1]);
    
    snd_mixer_elem_t* elem = snd_mixer_find_selem(handle, sid);

    snd_mixer_selem_get_playback_volume_range(elem, &min, &max);
    
    snd_mixer_selem_set_playback_volume_all(elem, volume * max / 100);

    snd_mixer_close(handle);

    return 0;
}