First of all : Quand parlé(s), although correct, is not the best way of saying this. In France (I don’t know about Quebec or other french-speaking countries though), the most idiomatic and frequent way of saying this is à l’oral. So you sentence would be :

Tous les mots en français me semblent être pareils à l’oral.

You can even improve the sentence by saying :

Tous les mots en français me semblent être pareils à l’oral.

But this is all another story. Now back to the topic.

Consider the same sentence in english :

All french words seem alike to me when spoken.

If you just used semblent without me, it would be exactly like if you removed the to me from the above sentence.
Which would make this statement a general truth : to everyone, french words seem alike.

However, what you mean is that to you – and only to you -, the words really sound the same. That’s why you have to refer to yourself using me.

Now the person you told this to could say :

J’ai parlé avec Marco, et il m’a dit que tous les mots français lui semblent pareils à l’oral.

He or she could also ask you :

Est-ce bien toi qui m’a dit que tous les mots français te semblent pareils à l’oral ?

So as you see, this has nothing to do with the verb se sembler, which doesn’t even exist.
This is just used to avoid making the sentence a general statement by “linking” it to the person for who it is true.

Here is all the words that can be used here depending on the subject :

• je -> me
• tu -> te
• il/elle -> lui
• nous -> nous
• vous-> vous
• ils/elles -> leur

I hope I helped you 😉