Here's updated instructions for 4.4:
1) Find these lines right near the top of the file:
$sort_fields = array(
'productcode' => 'lbl_sku',
'title' => 'lbl_product',
'price' => 'lbl_price',
'orderby' => 'lbl_default',
);
And right below that add:
# sort by manufacturer mod begins
if(!$manufacturerid)
$sort_fields["manufacturer"] = 'lbl_manufacturer'; #sort by manuf mod
# sort by manufacturer mod ends
2) At around line 905 there's a switch case. Right before these lines:
default:
$sort_string = "$sql_tbl[products].product";
Add:
# sort by manufacturer mod begins
case 'manufacturer':
$left_joins['manufacturers'] = array(
'on' => "$sql_tbl[manufacturers].manufacturerid = $sql_tbl[products].manufacturerid"
);
$sort_string = "$sql_tbl[manufacturers].manufacturer $direction";
break;
# sort by manufacturer mod ends
3) Make sure to run
WWW.YOURSITE.com/cleanup.php
That's it!