Modify the Soliloquy play/pause icons.

soliloquy-play-pause-buttons.css

/* Styles the Play button */
.soliloquy-controls-auto .soliloquy-controls-auto-item a.soliloquy-start { 
    width: 32px !important;
    height: 32px !important;
    background-color: rgba(0,0,0,0.7) !important;
    background-image: url('YOUR_IMAGE_URL') !important;
    background-size: 9px 14px !important;
}

/* Styles the Pause button */
.soliloquy-controls-auto .soliloquy-controls-auto-item a.soliloquy-stop { 
    width: 32px !important;
    height: 32px !important;
    background-color: rgba(0,0,0,0.7) !important;
    background-image: url('YOUR_IMAGE_URL') !important;
    background-size: 9px 14px !important;
}

/* Positions the play/pause button within the slider */
.soliloquy-container .soliloquy-controls-auto-item {
    width: 32px !important;
    height: 32px !important;
    position: absolute !important;
    bottom: 10px !important;
    right: 10px !important;
}