Though I'm fairly sure the Preta Path was done absorbing the fire, you make an interesting point. So if the Preta Path can't absorb genjutsu, then maybe ninjutsu is the only thing it can absorb.
That's exactly what it means.
From the time that Chakra Absorption was introduced to the appearance of Preta path, the only way for someone to absorb your chakra was through physical contact.
http://naruto.wikia.com/wiki/Chakra_Absorption_Techniques Let's take a look at Preta Path's ability.
http://naruto.wikia.com/wiki/Blocking_Technique_Absorption_Seal-----
Blocking Technique Absorption Seal* (Fuujutsu Kyuuin)
Ninjutsu, Fuuinjutsu, No rank, Defensive, Short range (0-5m)
User: Pain
An inexhaustible vortex, absorbing all techniques!!
By spinning the chakra flow within one's body in the opposite direction, all of the opponent's techniques aimed at oneself will be absorbed. The absorbed chakra is dispersed within the body. Like a bottomless swamp, the power is swallowed.
[picture of Pain's Preta Realm absorbing Jiraiya's Super-Big Ball Rasengan]
↑All techniques are absorbed and made ineffective, regardless of the
nature of the chakra.
-----
When talking about the nature of the chakra, they are referring to the elements (Fire, Water, Lightning, etc.). And when elemental chakra is involved, it's part of ninjutsu.
Correct me if I'm wrong, but Rinnegan users are the only people we have seen who are capable of absorbing chakra via this method.
Going back to the "Genjutsu Mullifying Ear Plugs", I see it's created by Mokuton. Even Mokuton requires physical contact to absorb chakra.
http://naruto.wikia.com/wiki/Wood_Release:_Foo_Dog_Headshttp://naruto.wikia.com/wiki/Wood_DragonSo even if Preta Path was capable of absorbing chakra from sound, Mokuton cannot do that.