I sometimes have small windows of time between contracts to help other musicians with this more complicated stuff. However, there is time and knowledge that goes into this that isn't worth being a free offer. I am willing to barter services to be defined later if the following apply.
• if your tracks are already clean and well-engineered
• if you have a clear idea of what sound you're going for up front or if you're just open to what comes out
• if you're a good communicator and in my moment during the work and available for quick turnarounds
• if you've given thought to some social cross-promotion you can do to help other projects
If all the above apply to you, I am open to helping some good musicians in need of help. PM me with what you're hoping to achieve and where you're at and I'll let you know if I can help. I prefer to master more than mix so hopefully the bounces aren't too crazy to start from.
I would also start making a plan for how you can monetize those that help or, as above, those charging minimal for their services. Its not that people want to do this for free because NO ONE DOES but the hope is that if you sell a few digital copies thanks to the mixing or mastering, that you be honest about it and offer some promotion and or some cash after. It's not FREE its DEFERRED PAYMENT.