I would prefer auscultare over audire, because it stands for attentive listening. Audire is the more general word and would probably not be wrong, though. Auscultare stands with the accusative or dative. As Unbrutal Russian points out in a comment, the dative is used when the sense is one of listening and believing, taking heed, doing as told. That does not fit here, so we want the accusative. And as TKR points out, the verb should not be in the imperfect tense. We are presumably talking about a listening event that antecedes (or goes along with) the reception of a t-shirt.
Instrumentum is neuter, so you can bet your infimum thalerum the plural is going to end in -a somehow. In this case, quite simply: instrumenta. That would be the accusative; the dative would be instrumentis. (At this point I was also going to complain about the plural ruinarum, but it turns out that that is quite common classically. So I learned something new today.)
There is nothing wrong with solum per se, although it would be a bit more usual to say ni(hi)l nisi here. Or, since in English you do not just say “only this t-shirt,” but “all I got etc,” one might prefer a more explicit phrase like nihil praeterquam.
Haec tunicam should be hanc tunicam.
Finally, dabar is wrong. I find the imperfect hard to justify here, but there is a more fundamental problem. You see, the recipient with dare is in the dative case, the thing given in the accusative. When you turn an active verb passive, the accusative object becomes the subject. The dative object just remains the dative object:
Tunicam mihi dedit (aliqui) = tunica mihi data est (ab aliquo)
So dabar means “I am given” all right, but in the sense that you are the thing that's changing hands. That makes little sense here. But never worry, Latin has your back with the nice word accipere.
So in sum I would suggest:
Instrumenta Ruinarum auscultavi et nihil accepi praeterquam hanc tunicam.