Is IRB approval required (by publishers or otherwise) for volunteer human subjects research?
Many journals have policies that require authors to have obtained IRB approval (or its equivalent in countries other than the US) as a condition of publication for any paper that involves human subjects. Here, the consequence of not obtaining IRB approval would be that your paper isn't published.   

If you are employed by an institution that has a policy on human subjects research than you may be required to obtain IRB approval for any research project involving human subjects by your institution's policy, whether or not the research is sponsored by the institution or makes any use of the resources of the institution.  Here, the consequences of not obtaining IRB approval could include being fired from your job.  

There are Commercial Institutional Review Boards that provide review and oversight for research involving human subjects outside of an institution with its own review board.  These are used, for example, by companies that do human subjects research as part of the development of new products.