What is the function of 所 in 聽吾所說心中所以?
聽吾所說心中所以

&quot;所以&quot; is reason (#4 definition in the below link).
http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000145637
roughly, i would interpret this verse as:
please listen to (聽) what i say (吾所說)
the reason (所以) in my heart (心中)
edited, in lieu of long comment.
the context is set, in 雜阿含經卷第二十三 [0168b13]
http://tripitaka.cbeta.org/ko/T02n0099_023

時﹒諸臣白王言﹒何故於此布施供養皆悉勝前﹒王曰﹒聽吾所說心中所以

at that time (時), statesmen asked the king (諸臣白王言) &quot;why (何故 ) donate (布施供養) more than before (皆悉勝前)?&quot;. the king replied (王曰) . . .
so, &quot;所以&quot; in this context should be interpret as reason. :)

therefore, listen with your heart to what I say

in english, this verse is ok. but, &quot;聽吾所說心中所以&quot; can't be. in literary chinese, such english verse would be:
故﹒汝當諦聽吾言
汝當諦聽 --&gt; listen with your heart
吾言 --&gt; what I say
故, 遂  --&gt; as conjunction &quot;therefore&quot;
edited again, in lieu of comment.
well, one need to read from preceding paragraphs:

[0167c25]　時﹒王生大歡喜﹒捨十萬兩珍寶﹒供養其塔﹒而說偈言 . . .
[0168a08]　時﹒王捨十萬兩珍寶﹒供養此塔﹒以偈讚曰 . . .
[0168a19]　時﹒王捨十萬兩珍寶﹒供養是塔﹒以偈讚曰 . . .
[0168a22] 　. . . 王曰：『以一錢供養。』. . .

then, for 阿難塔 [0168b04] :

[0168b13]　王即捨百億兩珍寶﹒而供養其塔。

十萬兩 is $100,000, while 百億兩 is $10,000,000,000; see the difference?
immediately next, is the sentence of our interest:

時﹒諸臣白王言﹒何故於此布施供養皆悉勝前

at that time (時), statesmen asked the king (諸臣白王言) :  &quot;why (何故 ) donate (布施供養) to this one (於此) more than before (皆悉勝前)?&quot;.
the king donated difference amount of money to various stupas (塔), ranged from 一錢 to 百億兩 ($10,000,000,000). those statesmen (諸臣) wanted to know why the king donate 百億兩 to 阿難塔 (the stupa of Ānanda), which was significantly more than other donations.
in english, &quot;donate + to&quot; is grammatically correct; so, &quot;於此&quot;, is roughly &quot;to this one&quot;.

王曰﹒聽吾所說心中所以

the king replied (王曰) : &quot;please listen to (聽) what i say (吾所說), the reason (所以) in my heart (心中)&quot;
the scenario is, statesmen asked why, the king replied, revealing the reason in his heart (mind).
about the suggested &quot;literally translation&quot;:

&quot;king say: 'listen i that which speaks heart in reason'&quot;

unfortunately, based on my experience, i would say nope. such suggestion is ignoring the context; and, well, un-literary-chinese, maybe un-chinese interpretation.
have fun :)