Additional Study Information: The purpose of this study is to evaluate the safety and effectiveness of a new combination treatment involving two drugs: selinexor and ixazomib, for the treatment of dedifferentiated liposarcoma, malignant peripheral nerve sheath tumor, alveolar soft part sarcoma and Ewing sarcoma. The primary objective of the study is to determine the maximum tolerated dose of the combination of the study drugs. These types of sarcoma are being studied because of earlier research that suggests that combination treatment with selinexor and ixazomib is most likely to be effective for these sarcomas.