The code for the sound effect of M size beam weapons docent have a proper muzzle sound reference, but it does have a proper impact reference sound as yo can see below.
<effect name="muzzle_gen_m_beam_01_mk1" alignment="object">
<elements>
<!-- Muzzle Smoke-->
<element id="1" ref="dummyemitter_up_macro" forkeffect="beam_muzzle_smoke_01" alignment="object" duration="10.0" minscale="1.5" maxscale="1.5" fadeout="1.0" forklod="none"></element>
<!-- Geometry -->
<element id="2" ref="muzzle_gen_m_beam_01_mk1_macro" forkeffect="none" duration="10.0" minscale="2.0" maxscale="2.0" forklod="none"></element>
<!-- Geometry Light -->
<element id="3" ref="impact_light_orange_macro" forkeffect="none" duration="10.0" minscale="5.0" maxscale="5.0" fadeout="1.0" forklod="none"></element>
<!-- Sound -->
<!-- <element id="3" ref="effect_dummy_macro" forkeffect="none" duration="1.5" forklod="none"> -->
<!-- <sound ref="wpn_impulse_player_muzzle"></sound> -->
<!-- </element> -->
</elements>
</effect>
<effect name="impact_gen_m_beam_01_mk1" alignment="normal">
<elements>
<!-- Impact Geometry -->
<element id="1" ref="impact_gen_m_beam_01_mk1_macro" forkeffect="none" duration="0.2" minscale="1.0" maxscale="1.0" fadein="0.0" fadeout="0.0" forklod="none"></element>
<!-- Impact Smoke Particles-->
<element id="2" ref="dummyemitter_macro" forkeffect="beam_impact_smoke_02" alignment="normal" duration="2.0" minscale="2.0" maxscale="2.0" forklod="none"></element>
<!-- Impact Fire Particles-->
<element id="3" ref="dummyemitter_macro" forkeffect="laser_impact_fire_01" alignment="normal" duration="1.0" minscale="1.0" maxscale="1.0" fadein="0.0" fadeout="0.5" forklod="none"></element>
<!-- Geometry Light -->
<element id="4" ref="impact_light_orange_macro" forkeffect="none" duration="5.0" minscale="5.0" maxscale="5.0" fadeout="1.0" forklod="none"></element>
<!-- Sound -->
<element id="4" ref="effect_dummy_macro" forkeffect="none" duration="1.0" forklod="none">
<sound ref="energy_beam_impact_loop"></sound>
</element>
</elements>
</effect>
Here is the code for the S version of the beam weapon effects for reference, and as you can see this one does have a proper muzzle sound ref.
<effect name="muzzle_gen_s_beam_01_mk1" alignment="object">
<elements>
<!-- Muzzle Smoke-->
<element id="1" ref="dummyemitter_up_macro" forkeffect="beam_muzzle_smoke_01" alignment="object" duration="10.0" minscale="0.4" maxscale="0.4" fadeout="1.0" forklod="none"></element>
<!-- Geometry -->
<element id="2" ref="muzzle_gen_s_beam_01_mk1_macro" forkeffect="none" duration="10.0" minscale="1.0" maxscale="1.0" fadeout="0.0" forklod="none"></element>
<!-- Geometry Light -->
<element id="3" ref="impact_light_red_macro" forkeffect="none" duration="10.0" minscale="1.0" maxscale="1.0" fadeout="0.8" forklod="none"></element>
</elements>
</effect>
<effect name="startshooting_gen_s_beam_01_mk1" alignment="object" emitterspace="zone">
<elements>
<element id="1" ref="effect_dummy_macro" duration="1">
<sound ref="wpn_beam_gen_startshooting" />
</element>
</elements>
</effect>
<effect name="stopshooting_gen_s_beam_01_mk1" alignment="object" emitterspace="zone">
<elements>
<element id="1" ref="effect_dummy_macro" duration="1">
<sound ref="wpn_beam_gen_stopshooting" />
</element>
</elements>
</effect>
THANKS IN ADVANCE!!!
~