Hello uriel and welcome to the world of XMOS. Audio is a key focus for XMOS but very likely you know this already :) For us, we are not involved with such audio processing but hope to help you to make a proper decision for your project.
XMOS has evolved with different controllers over the years. The latest versions are the XCORE-200 series which integrate USB & Ethernet (if you select the respective P/Ns) ports internal to the component. There are additional benefits and features to this new series and the best part is that they are relatively lower cost parts. If you select an older series device which are still in production (not aware of any EOL positions on the XS series) then you must apply external USB and/or Ethernet PHY devices to make such ports work for your project.
From some quick readings, you do have access to the XS series + add-on boards for the midi & audio in/out connections for example:
USB Audio DJ Kit
USB Audio DJ Kit
Part #: XR-USB-AUDIO-2.0-4C
Silicon on Board: XS1-U8A-64-BG96
* note the XS1 prefix = older but valid part to consider
However, XMOS is now advertising a new XCORE-200 kit for this topic (ETA is January but Digikey is showing stock already - perhaps the IP will follow later which is always being updated anyways):
XK-AUDIO-216-MC-AB
XK-AUDIO-216-MC-AB
Personally, we would lean towards the XCORE-200 kit since this uses the latest and integrated CPU from XMOS and may offer you the desired functions at a lower price point. However, do pay attention to the MFI license details. If you are not aware, you may need to apply for a license from Apple after their vetting process which is a small pain. In short, Apple will monitor your project, decide if it is worthy of production to release to you low level details to allow their platforms to be compatible and then monitor where you buy the raw parts. Yes, you must strictly buy parts specific to Apple through their chain of distribution. Also, if they feel that the design is good for Apple, they have the right to make their own version without any paid royalties to you. For example, connectors are to be sourced from a few suppliers they have approved as they are the official channels of distribution (ie. Avnet was one we recall). In addition, they have the right to audit your sales to confirm if they are to be paid some royalties and if there is a discrepancy then they will charge you for the audit and late penalties for any such payments. We did qualify and were granted the MFI license but we then declined their 'generous' offer to participate. Ironically, any breathing company in Asia is 'MFI licensed'. I am sure they all buy from their official chain of suppliers (end of sarcasm).
If the MFI license makes sense for you then do consider the route for your product line. If your are not MFI licensed then XMOS may not release the necessary source code to make the XCORE-200 based design operate. This will leave you with only a static (factory programmed) PCB. Yes, the CPU is based on an external or internal flash memory (based on the P/N of the CPU) so this PCB is a stand alone embedded design. No alternative technology on the market can challenge the speeds of XMOS devices. However, the drawback is the learning curve of the rather complex tools and the building of the custom IP for your project. Over the years, the IP has been growing so you have many examples to learn from.
Hope this helps but do review the options and hope someone from XMOS will chime in with more specifics on these kits. Not sure if the XTAG-3 board is included with the above kits as they are required - likely they are along with the required power supply. If not, the power supply is very low cost (around $10 USD or under but review the voltage range for each kit) and the XTAG-3 kit is $20 USD from Digikey as well.
Update:
Looks like the cables & power supply are included with this kit but perhaps not the XTAG-3 board which is another $20 USD:
User manual for XK-AUDIO-216-MC-AB
xCORE-200 Multichannel Audio Platform
http://www.xmos.com/support/boards?product=18334
This looks like a really solid kit for your application. Just confirm the MFI, XTAG-3 tool details and you should be good to go using the latest silicon.