This research study is testing whether an experimental drug, called SRD-001, is safe and helps the weakened heart of patients with Duchenne muscular dystrophy (DMD) regain its ability to effectively pump blood to the rest of the body. SRD-001 is a form of gene therapy. The goal of SRD-001 gene therapy is to provide the heart muscle cells with extra copies of the SERCA2a gene so that they can produce more SERCA2a protein to help the heart muscle cells squeeze/contract better. Researchers will compare SRD-001 treated participants with no-treatment participants; all participants will continue to take their current heart medications. All participants will be followed very closely for 2 years and undergo cardiac magnetic resonance imaging of their heart at baseline, year 1 and year 2 along with assessment of upper limb function and lung function. After the 2 years of close follow-up, all participants will roll over into long-term follow-up where they will be called biannually for information on their current medical status.