Cabe says:

Doubtful that the grammar is incorrect here, although the expected form is "it sounded strange."  To use the adverb gives the paralysis active force, personifies it.  In this sentence paralysis can act, and it acts strangely. 

The acting adverb tells us that Joyce is writing about something that has power.  It implies that this power is separate from the normal world -- the world of prescribed grammar for example.  It suggests that, to the boy at least, this power seems so fearful it is alive.

Thus the adverb is not just utilitarian (is not limited to the ironic or realistic levels).  There's a demonic quality to the paralysis: the kind that can affect the boy's dream world and his fears.  This paralysis that sounds "strangely" has meaning on the romantic or mythic levels.  Think about the implications of  that.

Consider: in most Christian views of the world, God is so powerful and omniscient that the Devil is included in God's purpose, as is the evil the Devil creates.  Any view that gives evil its own independent force separate from God is heretical.  Indeed, there is a heresy called the Manichean Heresy that tries to do just that. 

I am not arguing that Joyce is alluding to the Manichean Heresy that the Devil or Evil is equal to and separate from God, but I am pointing out that the adverb points to horror that fascinates, that almost takes on life when the boy speaks its name and hears the sound move in his ears. 

The boy repeats its name, in fascination.  Are we seeing something about him?