HL2 mod snippet - Looping sprint sounds

hl2_player.cpp

//Loop sprinting
bool running = (m_fIsSprinting && GetAbsVelocity().Length2D() > 40);
if( running || m_fSprintStart > 0.0f)
{
	EmitSound_t t;
	t.m_pSoundName = "Player.SprintLoop";
	t.m_nFlags = SND_CHANGE_VOL;

	if( m_fSprintStart > 0.0f && !running )
	{
		float diff = gpGlobals->curtime - m_fSprintEnd;

		//Fadeout under 1 seconds
		if( diff < 1.0f )
			t.m_flVolume = 1 - diff / 1.0f;
		else {
			m_fSprintStart = 0.0f;
			m_fSprintEnd = 0.0f;
			t.m_nFlags = SND_STOP;
		}
	} else {
		float diff = gpGlobals->curtime - m_fSprintStart;

		//Fadein under 3 seconds
		if( diff < 3.0f )
			t.m_flVolume = diff / 3.0f;
	}

	CPASAttenuationFilter filter( this );
	filter.UsePredictionRules();
	EmitSound(filter, entindex(), t );
}

AND HOW AND WHERE TO INSERT IT - I JUST NOT A PROGRAMMER