Going over this, the MIDI out and Line in ports are surface-mount whereas the main output jack is through-hole. Was this done because of space constraints?
Also, is this card's final BOM parts going to include color-coded jacks? Not to sound like a negative nancy on the subject, but even just color-coding the output jack will make things a lot nicer down the line for the end user as multiple jacks are going to be rather confusing.
You're also going to want to make up a PCB or 3D printable bracket for this card if anything given you're not going to sell these yourself past the initial small batch, I'm guessing.
But! I like where this is going, overall. I've been wondering when another 4237B card would come around. The only thing I don't like about the 4237B is it's not 100% SBPro compatible in my eyes, it has problems with some SCUMM games that have talkie elements (i.e. Beneath A Steel Sky) and that has led to a lot of hardship for me trying to keep such a card in one of my machines as I do enjoy playing them on bare metal from time to time. the 4237 by itself on a card really only belongs in a Win95/98/ME/2K/XP box where DOS game compatibility isn't everything. but with an OPL3 hooked to it, I'm sure it works well. I'd be interested to drop one of these in one of the slots on my 386SX boxes.
This is almost the perfect "kitchen sink" card for machines with only one or two ISA slots, as well-- the only thing it's missing is a DOS-compatible ethernet chipset plus a port with integrated magnetics for space savings. Easily solvable by use of other means (i.e. Xircom PE3) but given the PicoGUS already has Sound Blaster 2.0+OPL2 emulation going, one wonders if the 4237 is even needed. A re-spin without that, or even with, but focused on becoming an actual "kitchen sink" card that's everything here with the addition of an ethernet chipset might also be a lucrative side-project to take up. Such a card would allow someone to have everything under the sun necessary to run an old box right without expensive parallel port ethernet adapters, and then an extra slot saved from all of this could be used for a better graphics card instead of a network card. I've been waiting for someone to do such a project as I'm not knowledgeable EE-wise to undertake it myself, and will likely never have the time to get to that point.
However I do await the release of this to see where things go. I'm also interested to know what I'm assuming is an opamp up in the top right corner is? or is that just a headphone amp chip? I can't make out the model number to see. Based on the 4237 cards I have here I'm assuming it's a headphone amp chip.